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THE REFRACTION OF THE EYE 



By the same Author 

THE OPHTHALMOSCOPE: 

-L A Manual for Students. Third Edi- 
tion, with 4 coloured plates and 68 wood- 
cuts, 4*. 6d. 



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Surgeon, Presbyterian Hospital. Second Edition, Re- 
vised and Enlarged. 49 Illustrations. Cloth, net, $i oo 

The proper frame to be used and its correct adjustment are quite 
as important as the writing of the prescription for the glass ; an un- 
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The object of the book is to supply the latest and best knowledge 
upon a neglected subject, to inform the physician about matters 
that will promote the best result of his efforts to correct faults of 
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From The Medical Record^ New York. 

" The book is issued as an aid to those who prescribe and who 
sell eyeglasses and spectacles, for the purpose of enabling them 
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GOULD AJSTD PYLE. Compend of Diseases of the 
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tions, with a Section on Local Therapeutics. By 
GEOPGE M. GOULD. M.D., Author of Gould's Medi- 
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THE 



REFRACTION OF THE EYE 



A MANUAL FOR STUDENTS 



GUSTAVUS HARTRIDGE, F.R.C S. 

senior surgeon to the royal westminster ophthalmic hospital; 
ophthalmic surgeon and lecturer on ophthalmic surgery to 
the westminster hospital; ophthalmic surgeon to st. 
Bartholomew's hospital, Chatham ; consulting 
ophthalmic surgeon to st. george's dis- 
pensary, hanover square, etc. 



WITH ONE HUNDRED AND FIVE ILLUSTRATIONS 
AND SHEET OF TEST-TYPES 



Gentb IRevteeD JEfcition 



PHILADELPHIA 
BLAKISTON'S SON & CO. 

1012 WALNUT STREET 
I 90 I 



I90f 



9& N. Y. State Cytometric Asses. Jur^e 13, 10|l 



p* 

^ 






PBEEACE 



TENTH EDITION. 



In preparing the tenth edition of ' Refraction of 
the Eye' for publication, the original plan of the 
book has been maintained, and no effort has been 
spared to make the work more worthy of the favour 
with which it has been received in this country and 
abroad. 

Although but a short time has elapsed since the 
last large edition was brought out, every page has 
been carefully revised, and alterations made in 
accordance with our increasing knowledge of the 
subject. 

a. h. 

12, Wimpole Street, W.; 
March, 1900. 



PEEEACE 



FIRST EDITION. 



I have endeavoured in the following pages to state 
briefly and clearly the main facts with which practi- 
tioners and students should be acquainted, in order 
/ to enable them to diagnose errors of refraction accu- 
rately, and to prescribe suitable glasses for their 
correction. 

Those who would do this with facility can only 
acquire the requisite amount of dexterity by prac- 
tically working out a large number of cases of refrac- 
tion. No book, or even the knowledge gained by 
watching others who are thus employed, can take the 
place of this, the practical part of the subject. 

To many of my readers the chapter on Optics may 
appear unnecessary. I have added it for the benefit 
of those whose school education did not include this 
subject, since its elementary details so completely 
underlie the whole subject of refraction, that every 



Vlll PEEFACE 

student should understand them thoroughly before 
passing on to the real subject in hand. 

I have found it necessary in several instances to 
repeat important matters, and this I have done to 
obviate the necessity of continual references to other 
parts of the book, as well as in some cases to impress 
the importance of the subject upon the student. 

The woodcuts are numerous in proportion to the 
size of the work, but I consider that they are a very 
great help to the thorough understanding of the 
subject. 

The old measurements have been purposely omitted 
in favour of the almost universally adopted metrical 
system. It is confusing to the learner to have to do 
with two distinct sets of measurements, and no possible 
good can accrue from perpetuating- the old system of 
feet and inches. 

At the end of the work I have given a list of those 
authors to whom I have been indebted for much 
valuable information ; and, in conclusion, I take this 
opportunity of thanking my numerous friends for 
their help and suggestions. 

a. h. 

January, 1884. 



CONTENTS 



CHAPTER I 

PAGE 

Optics . . . . . .1 

Reflection . . . . .2 

Refraction . . . . .6 

Formation of Images . . . .17 



CHAPTER II 

Refraction of the Eye . . . .22 

Accommodation . . . .32 

Convergence . . . . .41 



CHAPTER III 

Methods of Determining the Refraction . 53 

Acuteness of Vision . . . .55 

Schemer's Method . . . .64 



CHAPTER IV 

The Ophthalmoscope . . . .66 

The Indirect Method . . . .66 

The Direct Method . . . .73 



CONTENTS 



Retinoscopy 



CHAPTER Y 



PAGE 
82 



CHAPTER YI 



hypermetropia . 
Aphakia . 



117 
132 



CHAPTER VII 



Myopia 



134 



CHAPTER YIII 



Astigmatism 
Anisometropia 



156 
184 



CHAPTER IX 



Presbyopia 

Paralysis of the Accommodation 
Spasm of the Accommodation 



187 
195 
197 



CHAPTER X 



Strabismus 



200 



CONTENTS XI 



CHAPTER XI 

PAGE 

Asthenopia . . . . 223 

Accommodative .... 225 

Muscular ... .227 

Retinal ... .233 



CHAPTER XII 

Spectacles ... . 236 

Cases . . . . . .242 

Appendix ..... 257 

Regulations for Army, Navy, &c. . 259 

Test Types . . . .261 



LIST OF ILLUSTRATIONS 



No. page 

1. Reflection by a plane surface . . . .2 

2. Virtual image formed by a plane mirror . . 3 

3. Reflection by a concave surface . . .4 

4. Ditto ditto . . .5 

5. Reflection by a convex surface . . .6 

6. Refraction by a plane surface . . .7 

7. Refraction by a prism . . . . . 8 

8. Ditto ditto . . . . .8 

9. Refraction by a spherical surface . . .9 

10. Ditto ditto . . . .10 

11. Formation of convex lenses . . . .11 

12. Different forms of lenses . . . .12 

13. Refraction of rays (secondary axes) by a convex lens . 13 

14. Refraction of parallel rays by a convex lens . . 13 

15. Ditto ditto . . .14 

16. Properties of a biconvex lens . . . .15 

17. Ditto ditto . . . .16 

18. Properties of a biconcave lens . . .16 

19. Refraction of parallel rays by a concave lens . . 17 

20. Formation of an inverted image . . .18 

21. Real inverted image formed by a convex lens . . 19 

22. Virtual image formed by a convex lens . . .19 

23. Virtual image formed by a concave lens . . 20 

24. Diagram of eye showing the cardinal points . . 23 

25. Formation of inverted image on the retina . . 25 

26. Emmetropic, hypermetropic, and myopic eyeballs . 27 

27. Eye represented by a biconvex lens . . .28 

28. Formation of visual angle . . . .29 

29. Diagram of accommodation . . . .35 

30. Scheiner's method of finding the punctum proximum . 36 

31. Amount of accommodation at different ages . . 40 

32. Diagram representing the convergence . . .43 



XIV LIST OP ILLUSTRATIONS 

No. 

33. Landolt's ophthalmodynamometer 

34. Diagram of the relative accommodation 

35. Angle subtended at nodal point by test type 

36. Scheiner's method .... 

37. Image formed in emmetropia by the indirect ophthalmo 

scopic method .... 

38. Image formed in hypermetropia 

39. Image formed in myopia 

40. Size of the image in emmetropia for different distances of 

the objective .... 

41&42. Decrease of the image in hypermetropia on withdrawin 

the objective .... 

43. Image formed in emmetropia . 

44. Image formed in hypermetropia 

45. Image formed in myopia 

46. Direct ophthalmoscopic examination in emmetropia 

47. Estimation of hypermetropia by the ophthalmoscope 

48. Estimation of myopia by the ophthalmoscope 

49. Rays coming from the hypermetropic eye 

50. Rays coming from the myopic eye 

51. Shadows in retinoscopy 

52. Image of a candle formed on the retina 

53 The image formed at different distances of the retina 

54. Real movements of the retinal image . 

55. Image formed in emmetropia 

56. Image formed in hypermetropia 

57. Image formed in myopia 

58. Image in myopia .... 

59. Image in hypermetropia 

60. The oblique shadow in astigmatism 

61. Cause of oblique shadow 

62. Shadow in retinoscopy . 

63. Band-like shadow in retinoscopy 

64. The amount of astigmatism as found by retinoscopy 

65. Retinoscopy with the plane mirror 

66. Refraction of a hypermetropic eye 

67. Refraction increased by changes in the lens 

68. Correction by a biconvex lens . 

69. Accommodation at different ages in a hypermetrope of 3 D. 



LIST OF ILLUSTRATIONS XV 
No. PAGE 

70. Refraction of a myopic eye .... 136 

71. Ditto ditto . . . .136 

72. Correction by a biconcave glass . . . 136 

73. Section of a myopic eyebull . . . . 139 

74. Accommodation at different ages in a myope of 2 D. , 140 

75. Size of retinal image in myopia . . . 145 

76. Section of cone of ligbt after passing through an astigmatic 

cornea ...... 159 

77. Diffusion patches when the cone is divided at rigbt angles . 159 

78. Interval of Sturm . . . . .160 

79. Simple hypermetropic astigmatism . . . 161 

80. Compound hypermetropic astigmatism . . 162 

81. Simple myopic astigmatism . . . . 162 

82. Compound myopic astigmatism . . . 162 

83. Mixed astigmatism .... 163 

84. Astigmatic clock face .... 170 

85. Astigmatic fan ..... 171 

86. Erect image of a disc seen through an astigmatic cornea . 172 

87. Same disc seen by the indirect method . . 172 

88. Tweedy's optometer . . . .179 

89. Diagram of the accommodation . . . 188 

90. Angle a in emmetropia .... 201 

91. Angle a in hypermetropia .... 202 

92. Angle a in myopia .... 202 

93. Diagram of primary and secondary deviation . . 204 

94. Strabismometer ..... 206 

95. Method of measuring the angle of the strabismus . 208 

96. Diagram representing convergent strabismus . . 210 

97. Diagram representing divergent strabismus . . 216 

98. Stereoscopic slide ..... 221 

99. Graef e's test for insufficiency of internal recti muscles . 231 

100. Scale for testing latent deviation at the reading distance . 232 

101. Convex and concave glasses acting as prisms . . 233 



Lithographic Plate opposite page 147 ; 

1, 2, and 3. Drawn from myopic patients. 

4. Copied from Atlas of Wecker and Jaeger. 
Test types .... 168,261 



THE REFRACTION OF THE EYE 



CHAPTER I 

OPTICS 

Light is propagated from a luminous point in every 
plane and in every direction in straight lines; these 
lines of direction are called rays. Rays travel with 
the same rapidity so long as they remain in the same 
medium. 

The denser the medium, the less rapidly does the 
ray of light pass through it. 

Rays of light diverge, and the amount of diverg- 
ence is proportionate to the distance of the point 
from which they come ; the nearer the source of the 
rays, the more they diverge. 

When rays proceed from a distant point such as the 
sun, it is impossible to show that they are not parallel ; 
and in dealing with rays which enter the eye, it will 
be sufficiently accurate to assume them to be parallel 
when they proceed from a point at a greater distance 
than 6 metres. 

A ray of light meeting with a body may be absorbed, 

1 



2 THE REFRACTION OF THE EYE 

reflected, or if it is able to pa?? through this foody it 
mar be refracted. 

Reflection 

Reflection by a Plane Surface 

Beflection takes place from any polished surface, 
and according to two laws. 

1st. The angle of reflection is equal to the angle 
of incidence. 

2nd. The reflected and incident rays are both in 
the same plane, which is perpendicular to the reflect- 
ing surface. 

Fig. l. 




Thus, if a b be the ray incident at B, on the mirror 
c P, and b e be the ray reflected, the perpendicular 
p b will divide the angle a b e into two equal parts, 
the angle a b p is equal to the angle r b e : while 
a b, p b, and e b lie in the same plane. 

\Yhen reflection takes place from a plane surface, 
the image is projected backwards to a distance behind 
the mirrow equal to the distance of the object in front 
of it, the image being of the same size as the object. 

Thus in Fig. 2 the image of the candle c is formed 
behind the mirror M, at c', a distance behind the 



REFLECTION 



mirror equal to the distance of the candle in front of 
it ; an observer's eye placed at e would receive the 
rays from c as if they came from c'. 



Fig. 2. 




M. The mirror, c. The candle, c'. The virtual image of the candle. 
E. The eye of the observer receiving rays from the mirror. 

The image of the candle so formed by a plane mirror 
is called a virtual image. 

Reflection by a Concave Surface 

A concave surface may be looked upon as made up 
of a number of planes inclined to each other. 

Parallel rays falling on a concave mirror are re- 
flected as convergent rays, which meet on the axis at 
a point (f, Fig. 3) called the principal focus, midway 
between the mirror and its optical centre c. The dis- 
tance of the principal focus from the mirror is called 
the focal length of the mirror. 



4 THE REFRACTION OF THE EYE 

If the luminous point be situated at r, then the 
diverging rays would be reflected as parallel to each 
other and to the axis. 

If the luminous point is at the centre of the con- 
cavity of the mirror (c), the rays return along the 
same lines, so that the point is its own image. 

If the luminous point be at a the focus Trill be at a, 

Fig. 3. 




and it is obvious that if the luminous point be moved 
to a, its focus will be at A ; these two points theref ore, 
a and a, bear a reciprocal relation to each other, and 
are called conjugate foci. 

If the luminous point is beyond the centre, its con- 
jugate focus is between the principal focus and the 
centre. 

If the luminous point is between the principal focus 
and the centre, then its conjugate is beyond the 
centre ; so that the nearer the luminous point ap- 
proaches the principal focus, the greater is the dis- 
tance at which the reflected rays meet. 

If the luminous point be nearer the mirror than the 



REFLECTION O 

principal focus (f), the rays will be reflected as diver- 
gent, and will therefore never meet : if , however, we 
continue these diverging rays backwards, they will 
unite at a point (h) behind the mirror ; this point is 
called the virtual focus, and an observer situated in 

Fig. 4. 




the path of reflected rays will receive them as if they 
came from this point. 

Thus it follows that — 

Concave mirrors produce two kinds of images or 
none at all, according to the distance of the object, as 
may be seen by looking at one's self in a concave mirror. 
If the mirror is placed nearer than its principal focus, 
then one sees an enlarged virtual image, which in- 
creases slightly in size as the concave mirror is made 
to recede; this image becomes confused and disap- 
pears as the principal focus of the mirror is reached : 
on moving the mirror still further away (that is be- 
yond its focus) one obtains an enlarged inverted 
image, which diminishes as the mirror is still further 
withdrawn. 



O THE EEFRACTTON OF THE EYE 

Reflection by a Convex Surface 

Parallel rays falling on such a surface are reflected 
as divergent, hence never meet ; but if the diverging 
rays thus formed are carried backwards by lines, 
then an imaginary image is formed which is called 
negative, and at a point called the principal focus (f). 

Foci of convex mirrors are therefore virtual ; and 
the image, whatever the position of the object, is 
always virtual, erect, and smaller than the object. 

Fig. 5. 




The radius of the mirror is double the principal 
focus. 

Refraction 

Effraction by a Plane Surface 

A ray of light passing through a transparent me- 
dium into another of a different density is refracted, 
unless the ray fall perpendicular to the surface sepa- 
rating the two media, when it continues its course 
without undergoing any refraction (Fig. 6, H k). 



REFRACTION 



A ray is called incident before entering the second 
medium, emergent after leaving it. 

A ray passing from a rarer to a denser medium is 
refracted towards the perpendicular; as shown in 
Fig. 6, the ray a b is refracted at b, towards the per- 
pendicular p p. 

In passing from the denser to the rarer medium 
the ray is refracted from the perpendicular ; b d is 
refracted at c, from p p (Fig. 6) . 

Fig. 6. 




Reflection accompanies refraction, the ray dividing 
itself at the point of incidence into a refracted portion 
b a, and a reflected portion b e. 

The amount of refraction is the same for any 
medium at the same obliquity, and is called the index 
of refraction; air is taken as the standard, and is 
called 1 ; the index of refraction of water is 1'3, that 
of glass 1*5. The diamond has almost the highest 
refractive power of any transparent substance, and 



8 THE REFRACTION Of THE EYE 

has an index of refraction of 2 '4. The cornea has an 
index of refraction of 1'3, and the lens 1*4. 

The refractive power of a transparent substance is 
not always in proportion to its density. 

If the sides of the medium are parallel, then all 
rays except those perpendicular to the surface which 
pass through without altering their course are re- 
fracted twice, as at b and c (Fig. 6), and continue in 
the same direction after passing through the medium 
as they had before entering it. 

If the two sides of the refracting medium are not 
parallel, as in a prism, the rays cannot be perpen- 
dicular to more than one surface at a time. 

Therefore every ray falling on a prism must un- 
dergo refraction, and the deviation is always towards 
the base of the prism. 

The relative direction of the rays is unaltered 
(Fig. 7). 

Fig. 7. Fig. S. 




If d M (Fig. 8) be a ray falling on a prism (a B c) at 
m, it is bent towards the base of the prism, assuming 
the direction M X; on emergence it is again bent at N ; 
an observer placed at e would receive the ray as if it 
came from k: the angle K B D formed by the two lines 



REFRACTION 9 

at H is called the angle of deviation, and is about half 
the size of the principal angle formed at A by the two 
sides of the prism. 

Refraction by a Spherical Surface 

Parallel rays passing through such a surface sepa- 
rating media of different density do not continue 
parallel, but are refracted, so that they meet at a 
point called the principal focus. 

If parallel rays k, d, e, fall on a b, a spherical sur- 
face separating the media M and N of which N is the 
denser ; ray d, which strikes the surface of A b at right 
angles, passes through without refraction, and is called 
the principal axis ; ray k will strike the surface at an 
angle, and will therefore be refracted towards the 
perpendicular c j, meeting the ray d at p ; so also with 
ray e, and all rays parallel in medium M. The point 
p where these rays meet is the principal focus, and the 
Fig. 9. 




distance between the principal focus aad the curved 
surface is spoken of as the principal focal distance. 
Eays proceeding from f will be paralled in M after 



10 THE REFRACTION OF THE EYE 

passing through the refracting surface. Bays parallel 
in medium N will focus at p' , which is called the ante- 
rior focus. 

Had the rays in medium M been more or less diver- 
gent, they would focus on the principal axis at a 
greater distance than the principal focus, say at H; 
and conversely rays coming from h would focus at G ; 
these two points are then conjugate foci. 

When the divergent rays focus at a point on the 
axis twice the distance of the principal focus, then its 
conjugate will be at an equal distance on the other 
side of the curved surface. 

If rays proceed from a point o, nearer the surface 
than its principal focus, they will still be divergent 
after passing through A b, though less so than before, 
and will therefore never meet ; by continuing these 

Fig. 10. 




rays backwards they will meet at l, so that the conju- 
gate focus of o will be at l, on the same side as the 
focus ; and the conjugate focus will in this case be 
spoken of as negative. 



LENSES 11 

Refraction by Lenses 

Refraction by lenses is somewhat more complicated. 

A lens is an optical contrivance usually made of 
glass, and consists of a refracting medium with, two 
opposite surfaces, one or both of which may be seg- 
ments of a sphere ; they are then called spherical 
lenses , of which there are six varieties. 

Fig. 11. 




1. Plano-convex, the segment of one sphere (Fig. 

11,B). 

2. Biconvex, segments of two spheres (Fig. 11, a). 

3. Converging concavo-convex, also called a con- 
verging meniscus. 

4. Plano-concave. 

5. Biconcave. 

6. Diverging concavo-convex, called also a diverg- 
ing meniscus. 

Lenses may be looked upon as made up of a number 
of prisms with different refracting angles — convex 
lenses, of prisms placed with their bases together ; 
concave lenses, of prisms with their edges together. 

A ray passing from a less refracting medium (as 



12 THE INFRACTION OP THE EYE 

air) through a lens, is deviated towards the thickest 
part, therefore the first three lenses, which are thickest 
at the centre, are called converging ; and the others, 
which are thickest at the borders, diverging. 

Via. 12. 




A line passing through the centre of the lens 
(called the optical centre), at right angles to the sur- 
faces of the lens, is termed the principal axis, and any 
ray passing through that axis is not refracted. 

All other rays undergo more or less refraction. 

Rays passing through the optical centre of a lens, 
but not through the principal axis, suffer slight devia- 
tion, but emerge in the same direction as they entered. 
These are called secondary axes (Fig. 13). The 
deviation in thin lenses is so slight that they are 
usually assumed to pass through in a straight line. 

Parallel rays falling on a biconvex lens are ren- 
dered convergent ; thus in Fig. 14 the rays A, B, c, 
strike the surface of the lens (l) at the points d, e, p ; 
the centre ray (b) falls on the lens at e perpendicular 
to its surface, and therefore passes through in a 
straight line; it also emerges from the lens at right 



BICONVEX LENSES 



angles to its opposite surface, and so continues its 
course without deviation ; but the ray a strikes the 
surface of the lens obliquely at d, and as the ray is 
passing from one medium (air) to another (glass) 

Fig. 13. 




Lens with secondary axes undergoing slight deviation. 

which is of greater density, it is bent towards the 
perpendicular of the surface of the lens, shown by the 
dotted line m k ; the ray after deviation passes through 
the lens, striking its opposite surface obliquely at o, 

Fig. 14. 




and as it leaves the lens, enters the rarer medium 
(air), being deflected from the perpendicular n o ; it 



14 THE REFRACTION OF THE EYE 

is now directed to h, where it meets the central ray 
b h ; ray c, after undergoing similar refractions, meets 
the other rays at h, and so also all parallel rays falling 
on the biconvex lens (l). 

Parallel rays, therefore, passing through a convex 
lens (l) are brought to a focus at a certain fixed point 

Fig. 15. 




(a) beyond the lens; this point is the 'principal focus, 
and the distance of this focus from the lens is called 
the focal length of the lens. 

Bays from a luminous point placed at the principal 
focus (a) emerge as parallel after passing through the 
lens. 

Divergent rays from a point (b) outside the princi- 
pal focus (f, Fig. 16) meet at a distance beyond (f') 
the principal focus on the other side of the lens (l), 
and if the distance of the luminous point (b) is equal 
to twice the focal length of the lens, the rays will focus 
at a point (c) the same distance on the opposite side 
of the lens ; rays coming from c would also focus at 
b : they are therefore called conjugate foci, for we 
can indifferently replace the image (c) by the object 
(b), and the object (b) by the image (c). 



BICONVEX LENSES 15 

If the luminous point (d) be between the lens and 
the principal focus (f), then the rays will issue from 
the lens divergent, though less so than before enter- 
ing it ; and if we prolong them backwards they will 

Fig. 16. 




meet at a point (h) further from the lens than the 
point D ; h will therefore be the virtual focus of d, and 
the conjugate focus of d may be spoken of as negative. 

Biconvex lenses have therefore two principal foci, 
f and f', one on either side, at an equal distance from 
the centre. 

In ordinary lenses, and those in which the radii of 
the two surfaces are nearly equal, the principal focus 
closely coincides with the centre of curvature. 

We have assumed the luminous point to be situated 
on the principal axis; supposing, however, it be to one 
side of it as at e (Fig. 17), then the line (e f) passing 
through the optical centre (c) of the lens (l) is a 
secondary axis, and the focus of the point e will be 
found somewhere on this line, say at p, so that what 
has been said respecting the focus of a luminous point 
on the principal axis (a b) is equally true for points 
on a secondary axis, provided always that the inclina- 



16 

r. : z. 



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1". :-f t:-: v::;ri 



7: IT 




7- " --, ,- ;,. -• ^ - 



f,-.-.--- 



7.- :? 




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FORMATION OF IMAGES 17 

continues in a straight line ; the raj A strikes the lens 
obliquely at d and is refracted towards the perpen- 
dicular, shown by the dotted line G h ; the ray after 

Fig. 19. 




deviation passes through the lens to k, where, on 
entering the medium of less density obliquely, it is 
refracted from the perpendicular o p, in the direction 
k m ; the same takes place with ray c at f and n ; so 
also with all intermediate parallel rays. 

Formation of Images. — To illustrate the formation 
of images the following simple experiment may be 
carried out : — Take a screen with a small perforation 
and place on one side of it a caudle, and on the 
other side a sheet of white cardboard at a suitable 
distance to receive any image : rays diverge from 
the candle in all directions, most of those falling 
on the screen are intercepted by it ; but some few 
pass through the perforation and form an image of 
the candle on the cardboard, the image being- in- 
verted because the rays cross each other at the 
orifice. It can further be shown that when the 
candle and cardboard are equally distant from the 

2 



18 THE REFRACTION OF THE EYE 

perforated screen, the candle flame and its image will 
be of the same size. If the cardboard be moved 
further from the screen the image is enlarged, if 
it be moved nearer it is diminished ; if we make a 

Fig. 20. 




dozen perforations in the screen, a dozen ima 

will be found on the cardboard, if a hundred then a 
hundred; but if the apertures are so close together 
that the images overlap, then instead of so many 
distinct images we get a general illumination of the 
cardboard. 

The image of an object is the collection of the foci 
of its several points ; the images formed by lenses 
are, as in the case of the foci, real or virtual. Images 
formed, therefore, by convex lenses may be real or 
virtual. 

In Fig. 21, let A b be a candle situated at an 
infinite distance ; from the extremities of a b draw 
two lines passing through the optical centre (c) of a 
biconvex lens, then the image of A will be formed 
somewhere on the line a c a (termed a secondary axis), 
say at a ; the image of b will be formed on the line 
b c b, say at h ; therefore b a is a small inverted image 



FORMATION OF IMAGES 19 

of tlie candle A b, formed at the principal focus of the 
convex lens. Had the candle been placed at twice 
the focal distance of the lens, then its inverted image 

Fig. 21. 




Real inverted image formed by convex lens. 

would be formed at a corresponding point on the 
opposite side of the lens, and would be of the same 
size as the object. 

If the candle be at the principal focus (f), then the 
image is at an infinite distance, the rays after refrac- 
tion being parallel. 

If, however, the candle (a b) be nearer the lens than 

Fig. 22. 




Virtual image formed by convex lens. 

the focus, then the rays which diverge from the candle, 
will, after passing through the convex lens, be still 



20 THE REFRACTION OF THE EYE 

divergent, so that no image is formed ; an eye placed 
at e would receive the rays from a b as if they came 
from a b ; a b is therefore a virtual image of a b, erect 
and larger than the object, and formed on the same 
side of the lens as the object. 

Images formed by biconcave lenses are always 
virtual, erect, and smaller than the object. Let a b 

Fig. 23. 




Virtual image formed by concave lens. 

be a candle, and f the principal focus of a biconcave 
lens ; draw from A b two lines through c, the optical 
centre of the lens, and lines also from A and b parallel 
to the axis; after passing through the lens they 
diverge and have the appearance of coming from a b, 
which is therefore the virtual image of a b. 

A real image can be projected on to a screen, but a 
virtual one can only be seen by looking through the 
lens. 



REFRACTION 21 



CHAPTER II 

REFRACTION. ACCOMMODATION. CONVERGENCE 

The eye may be looked upon as an optical instru- 
ment, a sort of photographic camera, designed to pro- 
duce by means of its refracting system a small and 
inverted picture of surrounding objects upon the 
retina; the stimulation produced by this picture on 
the retina is conveyed through the optic nerve to the 
ganglion cells of the cortex, that part known as the 
optical area ; this excitation of the ganglion cells 
becomes sensation, and thus it is here that the object 
seen comes within the domain of consciousness, 
and the brain interprets the retinal impressions 
transmitted to it. Immediately behind the trans- 
parent retina is a layer of pigment, which absorbs 
some of the rays of light as soon as the image is 
formed ; were this not so the rays would be reflected 
to other parts of the retina, and cause much dazzling, 
considerably interfering with vision ; this is the case 
in those persons who have a congenital absence of 
pigment, and who are known as albinos. 

The refracting system of the eye is so arranged, 
that very little spherical or chromatic aberration 



22 THE REFRACTION OF THE EYE 

takes place, as is the case with ordinary optical in- 
struments. 

For distinct and accurate binocular vision the fol- 
lowing conditions are necessary : 

1. That a well-defined inverted image be formed 
on the layer of rods and cones at the yellow spot of 
each eye. 

2. That the impression there received be conveyed 
to the brain. 

In a work of this character the first of these condi- 
tions alone concerns us, and for the carrying out of 
this — the media being transparent — three important 
factors call for a separate description, viz. 

Refraction. 

Accommodation. 

Convergence. 

Refraction. — This term is used to express the optical 
condition of the eye in a state of rest. There are 
three refracting surfaces in the eye — the anterior sur- 
face of the cornea, the anterior surface of the lens, 
and the anterior surface of the vitreous; and three 
refracting media — the aqueous, the lens, and the 
vitreous. These together make up the dioptric 
system, and may for the sake of simplicity be looked 
upon as equal to a convex lens of about 23 mm. focus. 
What was said about convex lenses applies equally to 
the eye as an optical instrument. 

A ray of light falling on the cornea does not, how- 
ever, follow the simple direction we might imagine 
when considering the eye merely as a lens of 23 mm. 



REFRACTION 23 

focus : the eye must be looked upon as a compound 
refracting system, composed of a spherical surface and 
a biconvex lens. To enable us to understand the 
course of a ray of light through the eye, it is necessary 
to be acquainted with the cardinal points of this com- 
pound system. Too much space would be occupied 
to explain how the position of these points is arrived 
at, but it suffices to say that, having first found the 
cardinal points of the cornea and then those of the 
lens, the cardinal points of the eye will be the result 
of these Wo systems together. 

The cardinal points of the eye are six in number, 
two principal points, two nodal points, and tivo prin- 
cipal foci. 

Fig. 24. 




In the above diagram of the emmetropic eye the 
cardinal points of this compound system are shown, 
all situated on the optic axis (f a) : at B are two prin- 
cipal points situated so closely together in the anterior 
chamber that they may conveniently be looked upon 



24 THE REFRACTION OF THE KYB 

- ne point : at N are two nodal points, also close 
together, — for simplicity we shall consider them as 
one point : at r is the first principal focus, at a the 
second principal focus. \Ve then have the following : 
c, the cornea ; l, the lens : m, the macula ; o, the optic 
nerve : e a. the optic axis ; b. the principal point ; x, 
the nodal point ; H, the centre of rotation of the eye, 
9'8 mm. in front of the retina : a. the second principal 
focus ; and f, the first principal f c 

The nodal points correspond nearly to the optical 
centre of the refracting system, the axis ray passing 
through these point? is not refracted : every ray 
directed to the first nodal point appears after refrac- 
tion to come from the second point, and continues in 
the same direction to that which it first had : the 
nodal points in the eye are situated about 7 mm. 
behind the cornea (Fig. 24. x). 

J : f vts. — When an incident ray pa— 3 

;gh the first principal point, the corresponding 
emergent ray passes through the second principal 
point, but the incident and emergent rays arc 
parallel ; the principal points are situated about 2 mm. 
behind the cornea (Fig. 24, b). 

The first principal us is that point on the 
where rays parallel in the vitreous meet : this point i- 
about 137 mm. in front of the cornea (Fig. 24, f). 

The secoii'i ifl that point on the 

where parallel rays meet after passing through the 
eye, 22*8 mm. behind the cornea > v Fig. 24, a). 

A luminous point placed above the principal i 
has it- image formed on the retina below tin's axis : and 



REFRACTION 25 

inversely, the image of a point below the principal 
axis will be formed above it. If we replace these 
two points by an object the same thing occurs, and we 
get an inverted image (Fig. 25) : it is essential that 
the method of formation of these inverted images be 
thoroughly understood. 

From every point of an object abc proceed diver- 
gent rays. Some of those rays coming from a, pass 
through the pupil, and being refracted by the dioptric 
system, come to a focus on the retina at a; some 
coming from b, focus at h, and some from c at c. In 

Fig. 25. 




the same way rays come from every part of the object 
as divergent rays, and are brought to a focus on the 
retina ; so that the retina, being exactly at the focal 
distance of the refracting system, receives a well- 
defined inverted image. 

Much has been said and written as to why images 
which are formed in an inverted position on the retina 
should be seen upright, and all sorts of ingenious 
explanations have from time to time been given. The 
whole thing is entirely a matter of education and ex- 



26 THE REFRACTION OF THE EYE 

perience, which is supplemented and corroborated by 
the sense of touch. "We have no direct cognizance of 
the image on the retina, nor of the position of its 
different parts, but only of the stimulation of the 
retina produced by the image; this stimulation is 
conducted by the optic nerve to the brain, producing 
there certain molecular changes. We do not actually 
see the retinal image, but the eye receives the rays 
emanating from the object looked at, and we refer the 
sensation in the direction of these rays ; thus, if an 
image is formed on the upper part of our retina, we 
refer the sensation downwards from which the rays 
must have come. 

The great advantage of inverted images is, that for 
a given-sized pupil a much larger retinal picture can 
be formed than would be the case if no inversion 
took place; for in the latter case all images must 
necessarily occupy a smaller space on the retina than 
the size of the pupil. 

The refraction of the eye is said to be normal when 
parallel rays are united exactly on the layer of rods 
and cones of the retina ; in other words, when the 
retina is situated exactly at the principal focal dis- 
tance of the refracting system of the eye. This 
condition is called emmetropia (tyu, within ; /mirpov, 
measure; w'^p, the eye) (Fig. 26, a). If parallel rays 
are focussed behind or in front of the retina, then the 
term ametropia (a, priv. ; pirpov, measure; w\p, the 
eye) is used, and of this there are two opposite 
varieties : 

Hypermetropia, when the eyeball is so short that 



REFRACTION 



27 



parallel rays are brought to a focus behind the retina 
(Fig. 26, b). 

Fig. 26. 




A. Emmetropic eye. B. Hypermetropic eye. c. Myopic eye. 

Myopia, when the eyeball is too long, so that parallel 
rays focus in front of the retina (Fig. 26, c). 

Emmetropia in a strict mathematical sense is very 
rare. 

If we represent the eye by a biconvex lens, and the 
retina by a screen ; then it will correspond to emme- 
tropia when the screen is situated at the principal 
focus of the lens, as e, Fig. 27 ; we represent hyper- 
metropia (h) by bringing forward the screen, and 
myopia (m) by moving it further away from the lens. 

In all eyes, vision ranges from the far point or 



28 THE REFRACTION OF THE EYE 

punctum remotum (which in the emmetropic eye is at 
infinity) to the near point or punctum proximum. 

Fig. 27. 




Convex lens ot 23 mm. focus. Parallel rays focus at E 
(emmetropia) on the screen, forming a well-defined image 
of the object from which rays come; at h (hypermetropia) 
they form a diffusion patch instead of an image. M (my- 
opia), also a diffusion patch, the rays having crossed and 
become divergent. 

The near point varies in the normal eye according 
to the amount of the accommodation, receding 1 ffradu- 
ally as age advances; when it has receded beyond 
22 cm. (which usually occurs in the emmetropic eye 
about the age of forty-five) the condition is spoken of 
as presbyopia. 

Infinity is any distance bej'ond six metres, the rays 
coming from a point at or beyond that distance being- 
parallel or almost so. 

The emmetropic eye, therefore, has its far point, or 
punctum remotum, situated at infinity ; the hyperme- 
tropic eye has its punctum remotum beyond infinity, 
and the myopic eye has its punctum remotum at a 
finite distance. 

Generally the two eyes are similar in their refrac- 



VISUAL ANGLE 29 

tion, though sometimes there is a very great difference. 
One eye may be hypermetropic, the other myopic; 
or one emmetropic, the other ame tropic. Anisome- 
tropia is the term used when the two eyes thus vary 
in their refraction. 

There may be differences also in the refraction of 
the different meridians of the same eye — astigmatism. 

In all forms of ametropia the acuteness of vision is 
liable to be diminished. The visual acuteness usually 
decreases slightly as age advances, without any dis- 
ease. 

The visual acuteness refers always to central vision. 
The yellow spot is the most sensitive part of the retina, 
and the sensibility diminishes rapidly towards the 
periphery. The acuteness is measured by the size of 
the visual angle, that is the angle formed at the pos- 
terior nodal point, which -point closely coincides with 
the posterior surface of the lens, and is about 15 mm. 
in front of the yellow spot. 

In Fig. 28, let c d be an object for which the eye is 
Fig. 28. 




accommodated. The lines c c, d d, drawn from the 
extremities of the object, cross at the nodal point N. 
The angle c n d will be the visual angle under which 



30 THE REFRACTION OF THE EYE 

the object c d is seen. The size of the angle depends 
upon the distance of the object as well as upon its 
magnitude, and the size of the image thus formed on 
the retina will also depend upon the antero-posterior 
length of the eyeball. 

Thus an object A b, which is as large as c d, but 
nearer to the eye, will be seen under a larger angle, 
the angle A x b being greater than the angle cnd. It 
is also clear that the image formed on the retina will 
be smaller at 1, when the antero-posterior diameter of 
the eye is less, as in hypernietropia, than it is at 2 in 
emmetropia, and that it will be larger in myopia, as 
at 3, where the eyeball is elongated. It is, therefore, 
easy to understand that a patient maybe able to read 
the smallest type and still have some defect of refrac- 
tion, unless the type be read at its proper distance 
(see Fig. 35). 

It is by the unconscious comparison of things 
of known size, and the amount of accommodation 
brought into play, that we are able to estimate the 
distance of objects, and not by the visual angle alone. 

Objects must therefore be of a certain size, and it 
has been proved that to enable us to see a complex 
figure like a letter distinctly, each part of the figure 
must be separated from the other parts by an interval 
equal to not less than the arc subtending an angle of 
r at the nodal point. 

It has been shown (Fig. 26, b) that in the 
hypermetropic eye in a state of rest, parallel rays are 
brought to a focus behind the retina, so that instead 
of a clear, well-defined image, we get a circle of dif- 



LENSES 31 

fusion. Convex glasses render parallel rays passing 
through them convergent, so that by placing a lens 
of the right strength in front of the hyperme- 
tropic eye, we bring forward its focus on to the retina. 

In myopia (Fig. 26, c) the focus for parallel rays 
is in front of the retina; concave glasses render 
parallel rays passing through them divergent, so that 
the proper concave glass will carry back the focus on 
to the retina. 

Lenses. — The lenses used for estimating the visual 
acuteness consist of two kinds, spherical and cylin- 
drical. Spherical lenses were until recently numbered 
according to their radii of curvature, which was 
considered as equal to their focal length in inches, 
a glass of 1-inch focus being taken as a standard. To 
this plan there were several objections. The standard 
glass being a strong one, weaker glasses had to be ex- 
pressed in fractions. Thus a glass of 4-inch focus was 
one fourth the strength of the standard 1 inch, and was 
expressed as j. In addition to the trouble and incon- 
venience of working with fractions, the intervals 
between the lenses were most irregular, and, moreover, 
the inches of different countries vary. At the Oph- 
thalmological Congress in 1872 it was decided to adopt 
a metrical scale of measurement. A lens of 1 -metre 
focus is taken as the unit, and is called a dioptre; a weak 
instead of a strong glass therefore becoming the unit, 
a lens of two dioptres is twice the strength of the 
former, and has a focal length of half a metre. Thus 
each lens is numbered according to its refracting 
power, and not, as in the old system, according to its 



32 THE REFRACTION OP THE EYE 

focal length ; so that we have a series composed of 
equidistant terms. The numbers 1 to 20 indicate the 
uniformly increasing power of the glasses. 

The focal length of a lens is not expressed in the 
dioptric measurement ; we have only to remember that 
it is the inverse of the refracting power, so that by 
dividing 100 cm. by the number of the lens we obtain 
its focal length in centimetres : for example, if the 
strength of the lens be 2 D., then the focal length will 
be 50 cm. ; if 10 D., then 10 cm. 

The intervals between dioptres is somewhat large, 
so that decimals, "25, '50, *75 of a dioptre, are intro- 
duced ; these work easily. 

Convex glasses magnify, and concave ones diminish 
the size of objects. 

The cylindrical lens still remains to be mentioned ; 
it consists of a lens one surface of which is usually 
plane while the other is the segment of a cylinder, 
and may be either convex or concave : if a convex 
cylinder be held vertically, the vertical meridian will 
be plane, exercising no influence on rays passing 
through it in that meridian; while the horizontal 
meridian will be convex, and will act as such on rays 
passing through it. The axis of the cylinders is 
usually indicated by a portion of the lens on each 
side being ground parallel to its axis. 

Accommodation. — In the normal eye, in a condition 
of complete repose, parallel rays come to a focus 
exactly on the rods and cones of the retina, and the 
object from which the rays come is therefore seen 
distinctly. 



ACCOMMODATION 33 

Kays from a near object proceed in a divergent 
direction, and come to a focns behind the retina ; the 
object would not then be clearly seen unless the eye 
possessed within itself the power of bringing rays 
which are more or less divergent into union on the 
retina. 

This power of altering the focus of the eye is called 
accommodation, and is due to an alteration in the form 
of the lens. That the eye possesses this power can 
easily be proved in many ways, apart from the con- 
scious muscular effort; perhaps as simple a way as 
any to demonstrate it to one's self is to look through 
a net held a short distance off at some distant object. 
Either the net or the object can be seen distinctly, 
but not both at once. If the meshes of the net be 
looked at, then the distant object becomes indistinct, 
and on looking at the object the meshes become con- 
fused. 

Accommodation, therefore, increases the refraction 
of the eye, and adapts it to near objects. The changes 
which take place in the lens during accommodation 
are — 

1st. The anterior surface becomes more convex and 
approaches the cornea. 

2nd. The posterior surface becomes slightly more 
convex, but remains the same distance from the cornea. 
That these changes take place may be proved in 
the following manner : — A lighted candle, or other 
convenient object, being held on one side of the eye, 
so as to form an angle of 30° with its visual axis, an 
observer looking into the eye from a corresponding 

3 



34 THE REFRACTION OF THE EYE 

position on the other side, will see three images of 
the flame : the first upright, formed by the cornea ; 
the second larger, upright, and formed by the anterior 
surface of the lens; the third smaller and inverted, 
formed by the posterior surface of the lens. When 
accommodation is put in force, images one and three 
remain unchanged in shape and position; image two, 
which is that formed by the anterior surface of the 
lens, becomes smaller, more distinct, and approaches 
image one, proving that this surface of the lens has 
become more convex and has approached the cornea. 
In an emmetropic eye adapted for infinity, it has been 
proved that the radius of curvature of the anterior 
surface of the lens is 10 mm. ; when accommodated 
for an object 13'5 cm. off it is changed to 6 mm. 

During accommodation the pupil becomes smaller, 
the central part of the iris advances, while the peri- 
pheral part slightly recedes. 

The alteration in the shape of the lens is due to 
the contraction of the ciliary muscle, which draws 
forward the choroid, thereby relaxing the suspensory 
ligament, and allowing the elasticity of the lens to 
come into play. This elasticity is due to the peculiar 
watch-spring arrangement of the lens fibres. 

When the ciliary muscle is relaxed, the suspensory 
ligament returns to its former state of tension, and so 
tightens the anterior part of the capsule, flattening 
the front surface of the lens.* 

* Another theory of accommodation is Tscheming's, whose experi- 
ments have led him to believe that when the ciliary muscle contracts 
it increases the tension of the zonula, and alters the lens surface from 
a spherical to a hyperboloid form. 



PUNCTUM PEOXIMUM 35 

When the muscle is relaxed to its uttermost, the 
lens has assumed its least convexity, and the eye is 
then adapted for its far point (punctum remotum) (r). 

In this condition the eye is spoken of as being in a 
state of complete repose. 

In the emmetropic eye the punctum remotum is 
situated at infinity. 

Fig. 29. 




Diagram of lens, cornea, &c. The right half is represented 
as in a state of accommodation, the left half at rest. 
A. The anterior chamber, c. The cornea. ~l. The lens. 
V. The vitreous humour, i. The iris. M. Ciliary muscle. 

When the ciliary muscle has contracted as much as 
it can, the lens has assumed its greatest convexity, 
and its maximum amount of accommodation is in 
force. The eye is now adapted for the nearest point 
which can be seen distinctly; this is called the punctum 
proximum (p). 

The position of the punctum proximum can be 
determined in several ways ; the ordinary plan is to 
place in the patient's hand the small test type, and 
note the shortest distance at which he can read No. 1 
with each eye separately. Or we may measure its posi- 
tion with the wire optometer, which consists of a steel 



36 THE REFRACTION OF THE EYE 

frame crossed by thin vertical wires ; this is supported 
in a handle to which a tape measure is attached ; the 
tape is placed against the temple, and held there while 
the frame is made gradually to recede from the 
patient's eye we are examining, stopping as soon as 
the wires become distinct, and reading off the number 
of centimetres on the measure. Another excellent 
plan by which to find the position of the punctum 
proximum is that of Schemer : close in front of the 
eye we wish to examine is placed a card pierced with 
two small pinholes, which must not be further apart 
than the diameter of the pupil; through these two 
holes the patient is directed to look at a pin held 
about one metre away (the other eye is of course ex- 
cluded from vision during the experiment) ; the pin 
will be clearly and distinctly seen. We then gradually 
approach it to the eye : at a certain place it will be- 
come double : the point at which the pin ceases to 
appear single will be the punctum proximum. 

In Fig. 30 the biconvex lens l represents the eye, 

Fig. 30. 




d the perforated card, p the pin, e e' the two sets of 
rays from p, which focus exactly at b, the retina. If, 



AMPLITUDE OP ACCOMMODATION 37 

however, the pin be brought nearer, so that the accom- 
modation is unable to focus the two sets of rajs, they 
will form, instead of one, two images of the pin on the 
retina as at a. These will be projected outwards as 
crossed images. 

The space between the punctum remotum and the 
punctum proximum is called the range of accommoda- 
tion. 

The force necessary to change the eye from its 
punctum remotum to its punctum proximum is styled 
the amplitude of accommodation. The amplitude of 
accommodation, therefore, is equal to the difference 
between the refracting power of the eye when in a 
state of complete repose, and when its maximum 
amount of accommodation is in force, and may be ex- 
pressed by the formula 

a = p — r. 

A convex glass placed in front of the eye produces 
the same effect as accommodation, i. e. it increases its 
refraction and adapts the eye for nearer objects. We 
can easily understand that the lens which enables an 
eye to see at its near point without accommodating 
is a measure of the amplitude of accommodation, 
giving to rays which come from the near point a 
direction as if they came from the far point. 

The amplitude of accommodation is much the same 
in every kind of refraction. If we wish to measure 
it in an emmetrope, we have merely to find the nearest 
point at which the patient can read small print. A 
lens whose focal distance corresponds to this is a 
measure of the amplitude of accommodation. Thus, 



38 THE REFRACTION OP THE EYE 

supposing 20 cm. the nearest distance at which he 
is able to read small print, we divide this into 100 cm. 
to find the focal distance of the lens (^^ = 5 D.); 
and in this case a lens of 5 D. is the measure of the 
amplitude of accommodation. 

Or we can measure it in an inverse manner by 
looking at a distant object through a concave glass ; 
the strongest with which we can see this distant 
object distinctly is the amplitude of accommodation, 
the concave lens giving to parallel rays coming from 
the distant object such an amount of divergence as 
if they came from a point situated at the principal 
focal distance of this glass. 

Therefore the amplitude of accommodation in 
emmetropia is equal to the refraction when adapted 
to its punctum proximum, and may be expressed by 
the formula 

a = p — oo* 
or a = p — 
or a = p 

The Accommodation of Hypermetropes. — A hyper- 
metrope requires some of his accommodation for dis- 
tant objects ; we must therefore, in order to find the 
amplitude of accommodation in his case, add on to 
the lens whose focal length equals the distance of the 
near point, that convex lens which enables him to see 
distant objects without his accommodation, and we 
express it by the formula 

" = v — {-'•)= P + >•• 

Thus, to take an example, we will assume the 

* oo is the sign for expressing infinity. 



AMPLITUDE OF ACCOMMODATION 39 

patient's near point to be 25 cm. (^^ = 4 T).), and 
that he has to use 4 D. of accommodation for distant 
objects; then the amplitude of his accommodation 
would be 4 D. + 4 D. = 8 D. 

a = 4P.- (-4D.) = 8D. 
The Accommodation of Myopes. — In a myope we 
have to subtract the glass which enables him to see 
clearly distant objects, from that whose focal length 
equals the distance of the near point. The formula 
will then be 

a = p — r. 

Thus, to find the amplitude of accommodation in a 
myope of 2 D., the near point being at 10 cm., we 
subtract from (^^ = 10) 10 D. the amount of the 
myopia, 2D., and the resulting 8 D. is therefore the 
amplitude of accommodation. 

a = lo D. — 2 D. = 8 D. 

Hence it is obvious that, with the same amplitude 
of accommodation, the near point is further away in 
hypermetropia than in emmetropia, and further in 
emmetropia than in myopia. Thus an emmetrope, 
with an amplitude of accommodation of 5 D., would 
have his near point at ( -i-JP- = 20) 20 cm. ; a hyper- 
metrope of 2 D., whose amplitude equalled 5 D., would 
require to use 2 D. of his accommodation for distance, 
leaving him 3D., which would bring his uear point to 
(J-0-0 = 33) 33 cm. ; and a myope of 2 D., who would 
require a concave glass of this strength to enable him 
to see at a distance, would have a near point of 14 cm. 
(1-00. _ 24) w ith the same amplitude. 



40 THE REFRACTION OF THE EYE 

Accommodation is spoken of as absolute, binocular, 
and relative. 

Absolute is the amount of accommodation which 
one eye can exert when the other is excluded from 
vision. 

Binocular, that which the two eyes can exert to- 
gether, being allowed at the same time to converge. 

Relative, that which the two eyes can exert to- 
gether for any given convergence of the visual lines. 
Fig. 31. 
Dioptres. 



Diagram Bbowing by the number of squares through which 
the thick lines pass, the amplitude of accommodation at 
different ages in emmetropia. The figures above represent 
the amount of accommodation ; those below, the near 
point ; and those on the left, the age of the individual. 

Fig. 31 diagrammatical!)- represents the amplitude 
of accommodation in emmetropia. 

As age advances the elasticity of the lens dimin- 
ishes, the accommodation therefore becomes less, and 
the near point gradually recedes. These changes 
commence at a very early age, long before the indi- 
vidual has come to maturity. 



CONVERGENCE 



41 



The following table gives the amplitude of accom- 
modation at different ages as shown in Fig. 89, 
p. 188. 

Years. Amplitude of accommodation 

10 . . . . . 14 D. 



15 








. 12 D. 


20 








10 D. 


30 








7D. 


40 








4-5 D. 


50 








2-5 D. 


60 








1 D. 


75 












Convergence. — This is the remaining element of dis- 
tinct binocular vision, and with this function the 
accommodation is very intimately linked, so that 
usually for every increase of the convergence a certain 
increase in the accommodation takes place. 

Convergence is the power of directing the visual 
axes of the two eyes to a point nearer than infinity, 
and is brought about by the action of the internal 
recti muscles. 

When the eyes are completely at rest, the optic axes 
are either parallel, or more usually slightly divergent. 
The angle thus formed between the visual and the 
optic axis is called the angle a, and varies according 
to the refraction of the eye. In emmetropia the angle 
is usually about 5°; in hypermetropia it is greater, 
sometimes as much as 7° or 8°, giving to the eyes an 
appearance of divergence ; in myopia the angle is 
less, often about 2°, or the optic axis may, even in 
extreme cases, fall on the inside of the visual axis, 



42 THE REFRACTION OF THE EYE 

when the angle a is spoken of as negative (p. 203) ; 
so that in myopia there is frequently an appearance 
of convergence,, giving one the idea of a convergent 
squint; hence the mere look of the patient's eyes 
with regard to their axes is not always quite reliable. 

The object of convergence is the directing of the 
yellow spot in each eye towards the same point, so as 
to produce singleness of vision; diplopia, or double 
vision, at once results when the image of an object 
is formed on parts of the retina which do not exactly 
correspond in the two eyes. 

To test the power of convergence prisms are held 
with their bases outwards. The strongest prism 
which it is possible to overcome, that is the prism 
which does not produce diplopia on looking through 
it at a distant object, is the measure of the converg- 
ence, and varies in different persons, usually between 
prisms of 20° and 30°, divided between the two eyes. 
This is the relative convergence for infinity. 

In considering convergence we have not only to 
bear in mind the condition of the internal recti 
muscles, but also the state of equilibrium produced 
by them and the action of their antagonists — the 
external recti. 

The nearer the object looked at, the more we have 
to converge, and the greater the amount of accom- 
modation brought into play. Hence, on converging 
to any particular point, we usually also involuntarily 
accommodate for that point, the internal recti and 
ciliary muscles acting in unison. 

Nagel has proposed a very simple and convenient 



CONVERGENCE 



43 



Fig-. 32. 




44 THE KEFRACTJON OF THE EYE 

plan, by which we may express the convergence in 
metres, calling the angle formed by the visual and 
median lines, as at m', the metrical angle. In Fig. 32 
E, e' represent the centres of rotation for the two eyes ; 
e h e' is the base line between the centres. When 
the eyes are fixed on some distant object, the visual 
lines are parallel or almost so, as e a, e' a"; the 
angle of convergence is then at its minimum, and 
the convergence is said to be adapted for its 
punctum remotum ; this then, being at infinity, is 
expressed C* = oo . 

If the eyes be directed to an object one metre 
away, the metrical angle E m' h equals one, i.e. C = 1 . 
If the object is 50 cm. off, then C = 2 ; if 10 cm., 
then (L^ = 10) C = 10. If the object had been be- 
yond 1 metre (our unit), but not at infinity, say 4 
metres, then = J. 

When the visual lines, instead of being parallel, 
diverge, then the punctum remotum is found by con- 
tinuing these lines backwards till they meet at c, 
behind the eye; the convergence is then spoken of 
as negative. 

When the eyes are directed to the nearest point at 
which they can see distinctly, say at m'", the angle of 
convergence is at its maximum, and it is said to be 
adapted for its 'punctum proximum. 

The distance between the punctum proximum and 
the punctum remotum is the range of convergence. 

The amplitude of convergence is the whole converg- 

# C is the sign for convergence. 



CONVEKGENCE 45 

ence that can be put in force, and we express it by 
the formula 

c = p — r. 

The punctum remotum of convergence is seldom 
situated at a finite distance : sometimes it is exactly 
at infinity, but in the majority of cases it is situated 
beyond infinity, i. e. the visual lines diverge slightly. 
In order to measure this divergence, and so obtain 
the punctum remotum of convergence, we place before 
the eyes prisms with their bases inwards (abducting 
prisms), and the strongest through which the person 
is still able to see singly is the measure of the nega- 
tive convergence. 

Prisms are numbered in degrees according to the 
angle of the prism. The deviation produced by a 
prism is equal to half its angle ; thus prism 8 will 
produce a deviation of the eye of 4°, and prism 20 a 
deviation of 10°. 

When a prism is placed before one eye, its action 
is equally divided between the two eyes. 

To take an example : if an abducting" prism of 8° 
placed before one eye (or what is the same thing, 4° 
before each eye) -be found to be the strongest through 
which a distant object can be seen singly, then each 
eye in our example has made a movement of diverg- 
ence equal to 2°, and the punctum remotum of con- 
vergence in this case is therefore negative, and is ex- 
pressed — 2°. By referring to the table on page 49 
it will be seen that when the centres of rotation of 
the eyes are 6*4 cm. apart, then the metre angle 



46 THE REFRACTION OF THE EYE 

equals l r -V>'. bo we reduce the 2 r to metre angles, 
thus : 

2° 120 

F50- = 110 = l09ma -> 

or it is sufficient to remember to divide the prism 
placed before one eve by seven ; thus in our example 
we should divide prism 8 C by seven, and this would 
give us approximately the same result. 

Another excellent plan for finding the punctum 
remotum of convergence is by Maddox's test, which 
consists of a small glass rod placed behind a stenopaic 
slit : when this is held horizontally before the right 
eye. and the rlame of a candle viewed from a distance 
of 6 metres with both eyes open, the left eye receives 
the image of the rlame, while the right receives the 
iniacre which is drawn out bv the rod into a Ions' 

a * ■ o 

vertical strip of light ; and since the image received 
bv the two eyes is very different, there is no tendency 
to fusion, and the eyes take up their position of rest. 
A suitable scale placed behind the candle will give us 
the amount of convergence or divergence in metre 
angles, according to the position occupied by the 
streak of light on the scale. Should the patient be a 
mvope or hypermetrope he should wear his correction 
when this test is applied. 

To find the punctum proximum of convergence, 
hold a prism, base outwards adducting prism), before 
one eve, and the strongest which can be so employed 
without producing diplopia, divided between the two 
eyes, givea the punctum proximum of convergence in 

Trees. But the accommodation must be stimulated 



CONVERGENCE 



47 



at the same time by means of a concave glass, other- 
wise we only obtain the relative punctum proximum. 
This can be reduced to metre angles as before. 

Or a simpler plan is to measure it with Landolt's 
ophthalmo-dynamometer, which is a small instrument 
consisting of a black metallic cylinder, a, made so as 
to fit upon a candle, b. The cylinder has a vertical 
slit *3 mm. in breadth, covered by ground glass : the 
candle being lighted, this slit forms a luminous line, 

Fig. 33. 




and will serve as a fixation object. A tape measure 
is conveniently attached, being graduated in centi- 
metres on one side, and on the other in the corre- 
sponding numbers of metre angles. 

To find the punctum proximum of convergence, the 
measure is drawn out to about 70 cm., its case being 
held beside one of the eyes of the patient, while the 
object of fixation is placed in the median line. If the 
illuminated line is seen singly, by pressing the knob 



48 THE REFEACTION OF THE EYE 

of the case the spring rolls up the tape, and the fixa- 
tion object is brought nearer the eve. So soon as the 
patient commences to see double, the nearest point of 
convergence is obtained, and the maximum of con- 
vergence is read off the tape in metre angles. This 
experiment should be repeated several times. 

In a normal case the minimum of convergence is 
usually about — 1 m a, the maximum 9 # 5 m a ; so that 
the amplitude of convergence equals 10'5 m a. 

"We know that the accommodation increases the 
nearer the object approaches, hence we see that both 
the convergence and accommodation increase and 
decrease together ; and in recording the convergence 
in the manner proposed by Xagel, it will be seen that 
in the emmetropic eye the number which expresses 
the metrical angle of convergence expresses also the 
state of refraction for the same point — this is a great 
advantage. Thus, when looking at a distant object, 
the angle of convergence is at infinity, C = x ; and 
the refraction is also at infinity, A = x . "When the 
object is at 1 metre, the angle of convergence = 1, 
and the amount of accommodation put into play 
= ID. "When the object is at 25 cm., then the angle 
of convergence = 4, and the amount of accommoda- 
tion = 4 D. 

The amplitude of convergence is somewhat greater 
than the amplitude of accommodation, passing it both 
at its punctum remotum and its punctum proximum. 

The following table shows the angle of convergence 
in degrees, for different distances of the object, when 
the eyes are 6*4 cm. apart : 







CONVERGENCE 




istauce of the object 
from the eyes. 


The metrical 
angle. 


Value expressed 
in degrees. 


1 metre 


1 


1° 50' 


50 


2111. 


2 


3° 40' 


33 


}■> 


3 


5° 30' 


25 


„ 


4 


7° 20' 


20 


» 


5 


9° 10' 


16 


)> 


6 


11° 


14 


>» 


7 


12° 50' 


12 


5> 


8 


14° 40' 


11 


>) 


9 


16° 30' 


10 


)t 


10 


18° 20' 


9 


>} 


11 


20° 10' 


8 


>> 


12 


22° 


7-5 


» 


13 


23° 50' 


7 


„ 


14 


25° 40' 


6-5 


}) 


15 


27° 30' 


6 


» 


16 


29° 20' 


5-5 


„ 


18 


33° 


5 


}} 


20 


36° 40' 



49 



Although accommodation and convergence are thus 
intimately linked together, it can very easily be 
proved that they may have a separate and indepen- 
dent action. If we suspend the accommodation with 
atropine, the convergence is not interfered with ; or 
an object at a certain distance being seen clearly 
without a glass, it can still be seen distinctly with 
weak concave and convex glasses, without any altera- 
tion of the convergence. 

It may, therefore, be stated that although the 
accommodation and convergence are intimately asso- 
ciated, they may be independent of each other to a 
certain degree, so as to meet ordinary requirements ; 
thus for instance, as age advances changes take place 

4 



50 



THE REFRACTION OF THE EYE 



in the lens which necessitate a stronger contraction of 
the ciliary muscle to produce the requisite change in 
the accommodation, while the convergence remains 
the same. 

It is obvious also that the relations between 
accommodation and convergence must necessarily be 
very different in ametropia, and this relation will be 
again referred to when treating the various errors of 
refraction in detail. 

The following diagram (Fig. 34) shows the relative 
amount of accommodation for different points of con- 

Fig. 34. 




vergcnce in an emmetrope aged fifteen. The amount 
of accommodation in excess of the metrical angle of 



RELATIVE ACCOMMODATION 51 

convergence is called positive, and the amount below 
negative. 

The diagonal d d represents the convergence from 
infinity to 5 cm. ; it is also a record of the accommo- 
dation. The line p p' p" indicates the maximum ac- 
commodation for each point of convergence, and the 
line r r the minimum. The numbers on the left 
and below the diagram are dioptres and metrical 
angles of convergence ; thus, when the visual lines 
are parallel, it will be seen that 3 # 5 D. of positive 
accommodation can be put into play, i. e. the object can 
still be seen distinctly with a concave glass of that 
strength ; 3*5 D. is therefore the relative amplitude 
of accommodation for convergence adapted to infinity ; 
or the metrical angle C being 5, which is a distance 
of 20 cm. away, the accommodation for that point 
would equal 5 D.; the positive amount that can be 
put in force while the angle of C remains the same 
would be 3 D., and the negative also 3 D., the object 
being seen clearly with a concave or convex glass of 
3 D., therefore the relative amplitude of accommoda- 
tion for C 5 is 6 D. When the angle C = 10 or any 
larger angle, the accommodation that can be put in 
force will be seen to be entirely on the negative side. 

Thus, the convergence being fixed, the amount of 
accommodation which can be brought into play for 
that particular point is the sum of the difference 
between the strongest concave and convex glass 
employed. 

The eye being accommodated for an object at a 
certain distance, the amount of convergence for that 



52 THE REFRACTION OP THE EYE 

particular point may be measured by placing in front 
of the eyes prisms, bases outwards; the strongest 
prism through which the object is still seen singly is 
the measure of the positive part of the amplitude of 
convergence. Prisms, bases inwards, give us the 
negative part — the sum of these is the amplitude of 
relative convergence. 



METHODS OF DETEEMINFNG THE EEFEACTION 53 



CHAPTER III 

METHODS OF DETEEMIXTNG THE EEFEACTION 

In entering npon the practical part of the subject 
the following subjective and objective methods present 
themselves for consideration. 

1. The acuteness of vision. 

2. Schemer's method. 

3. The ophthalmoscope. 

(a) The indirect method. 

(b) The direct method. 

(c) Retinoscopy. 

In every case we must proceed in a systematic 
manner, and before commencing to take the patient's 
visual acuteness, something may be gained by noticing 
the general appearance of the patient, the form of the 
face, head, etc. ; thus a flat-looking face is sometimes 
an indication of hypermetropia ; a head elongated in 
its antero-posterior diameter, with a long face and 
prominent nose, may indicate myopia. If the two 
sides of the face are not symmetrical, or if there be 
some lateral displacement of the nose from the median 
line, astigmatism may be suspected. We should also 
notice the shape of the eyes themselves, if large and 
prominent, or small ; in the former case we may sus- 
pect myopia, in the latter hypermetropia. Large 
pupils are suggestive of myopia, and small pupils of 



54 THE REFRACTION OF THE EYE 

hyperrnetropia. In high degrees of astigmatism it can 
sometimes be seen that the curvature of the cornea in 
one meridian exceeds that of the other. The distance 
between the eyes should also be noted, as well as the 
direction of their visual axes. 

We next listen to the patient's own statement of the 
troubles from which he suffers ; he may say that he 
sees distant objects well but has difficulty in reading, 
especially in the evenings, or that after reading for 
some time the type becomes indistinct, so that he must 
rest awhile, — here we suspect hyperrnetropia; or he 
may be able to read and do near work, but sees badly 
at a distance, — then we suspect myopia ; or both near 
and distant vision may be defective, — in this case our 
first object must be to decide whether the imperfect 
vision is due to some error of refraction or to some 
structural change in the eyes themselves ; and we 
possess an extremely simple method by which to 
differentiate between them, and this method is called 
the Pin-hole test. 

Pix-hole Test. — A black diaphragm having a small 
perforation in its centre (the box of trial glasses 
usually contains such a diaphragm) is placed quite 
close to the eye under examination. This perforation 
gives passage to a small pencil of rays which passes 
through the axis of the refracting system of the eye, 
so that the image formed is clearly defined for all 
distances : if then the pin-hole improve vision, the 
refractive system is at fault ; but if, on the contrary, 
vision is not improved, then we suspect that the 
transparency of the media or that the retinal sensi- 



ACUTENESS OP VISION 55 

bility is defective ; thus we possess a very simple and 
reliable plan, which if used systematically, may save 
much loss of time. The points to notice when apply- 
ing this test are, that the illumination is good, and 
that the pin-hole is immediately in front of the centre 
of the pupil. 

Having then found out that the patient's ref faction 
is defective, we proceed to the first method, the 
acuteness of vision. 

The Acuteness of Vision. — This must not be confused 
with the refraction, and it is necessary clearly to 
understand the difference between these two terms. 
The acuteness of vision is the function of the nervous 
apparatus of the eye, while the refraction is the func- 
tion of the dioptric system ; so that the acuteness of 
vision may be normal, even if the refraction be very 
defective, provided it has been corrected by glasses. 
The refraction, on the other hand, may be normal, 
even though the eye is unable to see, as in cases of 
optic atrophy, etc. 

We may define the acuteness of vision as that degree 
of sight which an eye possesses after any error of its 
refraction has been corrected, and the glasses neces- 
sary for this correction are therefore a measure of the 
error of refraction. 

In order to find out the acuteness of vision, we have 
to determine the smallest retinal image the form of 
which can be distinguished ; it has been discovered by 
experiments that the smallest distance between two 
points on the retina which can be separately perceived 
is 0*00436 mm., about twice the diameter of a single 



56 



THE REFKACTION OF THE EYE 



cone ; but it is only at the macula and the part imme- 
diately around it, which is the most sensitive part of 
the retina, that the cones are so close together as 
•002 mm. ; in the periphery of the field of vision the 
two points must be further apart to appear distinct. 

It is probable that rays from two points must fall 
upon two different cones in order to be visible as two 
distinct objects. 

The smallest retinal image which can be perceived 
at the macula corresponds to a visual angle of 1', so 
that two stars separated by an angular interval of less 
than V would produce upon the eye the effect of one 
star only. 

The visual angle has been shown to be an angle 
included between two lines drawn from the two oppo- 
site edges of the object through the nodal point (Figs. 
28 and 35). 

Fig. 35. 




Test-types have been constructed upon these prin- 
ciples for determining the acuteness of vision, Snel- 
len's being those ordinarily used. Every letter is so 
made that when at its proper distance, each part of it 
is separated from the other parts by an interval equal 



ACUTENESS OF VISION 57 

to not less than the arc subtending an angle of Y at 
the nodal point, while the whole letter subtends an 
angle of 5'. 

In order to estimate the refraction by the acnteness 
of vision, the test object must be placed in a good 
light, and so far away as to exclude as much as pos- 
sible the accommodation, — 6 metres has been found to 
be a sufficient distance ; rays coming from an object 
so far off may be assumed to be parallel, and falling 
on an emmetropic eye at rest, would come to a focus 
on the retina. The smallest letter which can be seen 
distinctly at this distance will represent the patient's 
vision. 

Snellen's type consists of rows of letters, each being 
marked above with the distance in metres at which it 
should be read. The top letter should be distinct at 
60 metres, the next at 36, and each succeeding row at 
24, 18, 12, 9, and 6 metres respectively.* The patient 
placed at six metres should, without any accommoda- 
tion, be able to read the bottom line with either eye. 
This is expressed in the form of a fraction, in which 
the numerator indicates the distance at which it is read, 
and the denominator the number of the line. We note 
down the result found for each eye separately : if the 
bottom line is read, |- expresses it ; if the next, -| ; the 

^P. e 6 o; etc - 

If our patient, however, be not able even to see 

* The set of test-types at the end of the book has an additional line, 
and is marked 5, so that a greater amount of visual acuteness than £ 
can be estimated, and is, of course, recorded -§. Sets of type are now 
made which go down to f . 



56 



THE REFRACTION OF THE EYE 



cone ; but it is only at the macula and the part imme- 
diately around it, which is the most sensitive part of 
the retina, that the cones are so close together as 
•002 mm. ; in the periphery of the field of vision the 
two points must be further apart to appear distinct. 

It is probable that rays from two points must fall 
upon two different cones in order to be visible as two 
distinct objects. 

The smallest retinal image which can be perceived 
at the macula corresponds to a visual angle of V , so 
that two stars separated by an angular interval of less 
than V would produce upon the eye the effect of one 
star only. 

The visual angle has been shown to be an angle 
included between two lines drawn from the two oppo- 
site edges of the object through the nodal point (Figs. 
28 and 35). 

Fig. 35. 




Test-types have been constructed upon these prin- 
ciples for determining the acuteness of vision, Snel- 
len's being those ordinarily used. Every letter is so 
made that when at its proper distance, each part of it 
is separated from the other parts by an interval equal 



ACUTENESS OF VISION 57 

to not less than the arc subtending an angle of V at 
the nodal point, while the whole letter subtends an 
angle of 5'. 

In order to estimate the refraction by the acuteness 
of vision, the test object must be placed in a good 
light, and so far away as to exclude as much as pos- 
sible the accommodation, — 6 metres has been found to 
be a sufficient distance ; rays coming from an object 
so far off may be assumed to be parallel, and falling- 
on an emmetropic eye at rest, would come to a focus 
on the retina. The smallest letter which can be seen 
distinctly at this distance will represent the patient's 
vision. 

Snellen's type consists of rows of letters, each being 
marked above with the distance in metres at which it 
should be read. The top letter should be distinct at 
60 metres, the next at 36, and each succeeding row at 
24, 18, 12, 9, and 6 metres respectively.* The patient 
placed at six metres should, without any accommoda- 
tion, be able to read the bottom line with either eye. 
This is expressed in the form of a fraction, in which 
the numerator indicates the distance at which it is read, 
and the denominator the number of the line. "We note 
down the result found for each eye separately : if the 
bottom line is read, -§- expresses it ; if the next, -§ ■ the 
to P, to, etc. 

If our patient, however, be not able even to see 

# The set of test-types at the end of the book has an additional line, 
and is marked 5, so that a greater amount of visual ncuteness than £ 
can be estimated, and is, of course, recorded f. Sets of type are now 
made which go down to •§. 



58 THE RE FRACTION OF THE EYE 

the large letter at tlie top, we allow him to approach 
the board, telling him to stop as soon as the letter 
becomes visible. Supposing he stop at 2 metres from 
the board, we express that as -^ ; if he is not able to 
read it at all, we see how far off he can count fingers. 
If unable to do this, a lower degree of visual acuteness 
is found out by determining the ability to distinguish 
different sorts of light, as to colour, etc. This is called 
" qualitative perception of light," whereas a still 
lower degree is to distinguish the difference between 
light and darkness ; this is " quantitative perception 
of light." 

Although the capability of reading the bottom 
line at 6 metres is the average acuteness of vision, 
yet it is not the maximum, since many young people 
will be found who are able to read line six at 7 
metres, or even further, in which case their acute- 
ness is -J. 

Savages often have an acuteness of vision much in 
excess of the normal. 

Thus we have a standard of normal vision, and a 
convenient method of expressing it in a numerical 
manner. 

We put our patient then, with his back to the light 
in front of the test-types, which must hang well illu- 
mined at 6 metres distance, and having armed him 
with a pair of trial frames, we exclude the left eye 
from vision by placing in front of it a ground glass 
disc, and proceed to test the right eye by asking him 
how much of the type he is able to read ; if he read 
the line marked G, then his vision is £ or 1, that is to 



ACUTENESS OF VISION 59 

say, his distant vision is normal ; we may, therefore, 
assume the absence of myopia or astigmatism; but 
he may have hypermetropia, and only be able to read -| 
by using his accommodation ; this we decide by hold- 
ing a weak convex glass (+ *5 D.) in front of the eye, 
when if he still be able to read the same line f, he 
has hypermetropia, and the strongest convex glass 
with which |- can be read is the measure of the mani- 
fest hypermetropia ; supposing + 1 D. the strongest 
glass with which -| can be read, then we record it 
thus : R. V. £ Hm. 1 D. = f 

I say manifest hypermetropia, because in all cases 
occurring in young people this is not the total hyper- 
metropia; for in these a great part of the error is latent, 
which can only be discovered by using atropine, or by 
estimating the refraction by the direct ophthalmo- 
scopic method. Many cases will come before us 
having two or three dioptres of hypermetropia, who 
complain that the weakest convex glass impairs 
distant vision; in these cases the hypermetropia is 
wholly latent. 

We may say, therefore, that a patient who is able 
to read |- with one eye, must be — 
Emmetropic 
or 
Hypermetropic in that eye. 

If hypermetropic, a part of it is usually manifest, as 
found out by the strongest convex glass which does 
not impair distant vision; or it may be wholly latent, 
when it is necessary to atropise the patient before we 
can demonstrate it. 



60 THE REFRACTION OF THE EYE 

Supposing, however, our patient's vision is below 
the normal, and, instead of reading -|, he is only able 
to read, say the third line ( T 6 T ), and that this is blurred 
with a weak convex glass, he may have — 

Myopia, 

Astigmatism, 
or 

Spasm of accommodation (see p. 197). 
We try if a weak concave glass helps him ; if it does 
so, the case is one of myopia; and we find the weakest 
concave glass with which he sees best ; thus to take 
an example in which the patient is a myope and sees 
only ■£%, but with — 2D. reads -§-; we repeat the exami- 
nation with the second eye, and record it — 
B.T.A-2D. = f. 
L.V.A-2D. = f. 
It our patient is not improved with concave glasses, 
then we assume that some astigmatism is present, 
presupposing of course that there is no other cause 
for bad vision. 

To estimate this astigmatism we must call to our 
aid some of the methods described in the chapter on 
astigmatism, p. 156, or we may find out the spherical 
glass with which he is able to see best, then rotate in 
front of it a weak convex cylindrical glass, starting 
with its axis vertical; no improvement occurring, we do 
the same with a weak concave cylinder, starting with 
its axis horizontal ; finding by this plan the glass and 
its particular axis which gives the best result. It is 
necessary that the eye be thoroughly under the 
influence of atropine, in order to enable us to arrive 



ACUTENESS OF VJSION 61 

at definite and reliable results by this method. With 
practice, one is able in this way to work out simple 
cases of astigmatism accurately and quickly. 

The object in view is always to bring up the vision 
of each eye as nearly to the normal standard of f as 
possible. Frequently, however, we have to be 
satisfied with -| or ^-. 

But should the case appear to be a difficult one, it 
is better perhaps for the student not to waste time, 
but proceed at once to retinoscopy. 

When trying the patient at the distant type it is 
convenient to have two or more sets of letters, so that 
the type may be changed when the patient gets accus- 
tomed to one set. 

The near type is chiefly used to estimate the 
accommodation, by finding out the far and near point 
at which any particular line is read. Snellen's and 
Jaeger's are the types most commonly in use, many 
preferring Jaeger's, owing to the letters being of the 
ordinary shapes ; but they have the disadvantage that 
they are not arranged on any scientific plan, but are 
simply printer's types of various sizes : the set of 
reading type at the end of the book is so arranged 
that when held at the distance for which it is marked, 
each letter subtends an angle of 5' at the nodal point. 

It must, however, be remembered that sentences are 
an inferior test to letters, many people recognising 
the words by their general appearance, whereas they 
may be unable to see distinctly each letter of which 
the sentence is composed. 

Having tested our patient's vision at the distant 



62 THE REFRACTION OF THE EYE 

type and recorded the result) we place in his hand the 
reading type, and note the smallest print he is able to 
read and the distance at which he reads it ; first with 
each eye separately, then with the two together. 

In cases of myopia we may thus get a Valuable 
hint as to the amount of the defect : we will take for 
an example a case in which the patient can read -^ 
with the right eye ; we give him the near type, and if 
he can read the smallest only by holding it at a 
nearer point than the distance for which it is marked, 
we note the greatest distance at which he is able to 
read it ; if the type marked for 1 metre cannot be read 
further off than 25 cm., our patient has then most 
likely myopia of 4 D., because 25 cm. is probably his 
far point. In this case a glass — 4 D. would give to 
rays coming from a distant point the same amount of 
divergence as if they came from 25 cm. (-^^ = 4). 

We try the patient at the distant type with — 4 
D. j if he now read -| the myopia is confirmed, and the 
weakest glass with which he reads it is the measure 
of his myopia. 

If the patient read -|, but be unable to read the near 
type except it be held at a further distance than that 
for which it is marked, the case is one of paralysis of 
the accommodation, or presbyopia ; and as the latter 
only commences in emmetropia about the age of forty- 
five, it will be clear according to the age of the patient 
to which division the case belongs. 

As objects seen through convex glasses appear en- 
larged, and through concave glasses diminished, it 
follows that these, when placed before the eye, will 



ACUTENESS OE VISION 63 

exercise the same influence on the size of the retinal 
image. Noav the hypermetropic eye sees objects 
smaller, and the myopic eye larger than the emme- 
trope, and if glasses which are to correct the ame- 
tropia be placed at the anterior focal point, i. e. about 
13 mm. in front of the cornea, the retinal image of 
the ametrope will be of the same size as that of the 
emmetrope. 

Before leaving this subject of the acuteness of 
vision the following directions may be given : 

1st. The test-type must be in a good light; the 
advantage of artificial illumination is that it is uni- 
form. 

2nd. Commence with the right eye, or that which 
has the best vision, covering up the other with an 
opaque disc placed in a spectacle frame; do not be 
contented to allow the patient to close one eye, as he 
may not do so completely, or he will probably uncon- 
sciously slightly diminish the palpebral aperture of 
the eye under examination, whereby the circles of 
diffusion may be somewhat diminished and so give 
misleading results. Neither should he close the eye 
with his hand, he may look between the fingers, or 
exercise some pressure, however slight, on the eye- 
ball, which may interfere temporarily with the func- 
tion of the retina and so cause delay. 

3rd. Having noticed what each eye sees without 

glasses, always begin the examination with convex ones, 

so as to avoid calling the accommodation into action. 

4th. Having recorded the result found for each 

eye separately, we try the two together, the binocular 



64 THE REFRACTION OF THE EYE 

visual acuteness being usually slightly greater than 
that for one eye. 

5th. Test the patient with the reading type, noting 
the farthest point at which the smallest type can be 
read. 

Scheiner's Method. — Although this plan for detecting 
ametropia is now but little used, it is necessary the 
student should understand the principles upon which 
it is based. A diaphragm having two small perfora- 
tions is placed in front of the eye we wish to examine ; 
the perforations must be so near together that rays 
passing through them will enter the pupil (Fig 36). 
The patient is directed to look at a small ilame 6 
metres off; rays emanate from this flame in all direc- 
tions, some fall on the diaphragm, the greater number 
are thus cut off, but a few rays pass through the two 
openings, and if the eye be adapted for the flame, 
i. e. if it is emmetropic, these two sets of rays will 

Fig. 36.* 




meet exactly on the retina, forming there one image 
of the flame (b, Fig. 36) ; if, however, the eye be 

* In the above diagram, P is represented as a near object with rays 
diverging from it; it should be a distant object with parallel rays. 



scheiner's method 65 

hypermetropic (with suspended accommodation), then 
the two sets of rays will reach the retina before 
meeting, each set forming an image of the flame (a, 
Fig. 36) . The greater the hypermetropia the further 
apart will the images be formed ; these are projected 
outwards as crossed images, and the patient has there- 
fore crossed diplopia. That convex glass (from our 
trial box) which, held behind the diaphragm, causes 
the flame to be seen singly, is a measure of the hyper- 
metropia. If the eye be myopic, then the two sets of 
rays will have crossed and are diverging when they 
reach the retina, where two images of the flame are 
therefore formed (c, Fig. 36). These images are 
crossed again as they are projected outwards, and 
having twice crossed, homonymous images are the 
result. To find the amount of myopia, we have only 
to find the concave glass which, placed behind the dia- 
phragm, brings the two images into one. To enable us 
to tell if the images are crossed or homonymous, it is 
usual to have in front of one of the perforations a piece 
of coloured glass. We will suppose the diaphragm held 
so that the two openings are horizontal, that to the 
patient's right having in front of it a piece of red glass : 
if only one flame is seen the case is one of emmetropia ; 
if two images of it appear, one white, the other red, 
with the red to the left of the other, the images are 
crossed, and the case is one of hypermetropia. If the 
red appear on the right, then the case is one of 
myopia. The further apart the images are, the 
greater is the ametropia. 



68 THE EEFRACTION OP THE EYE 

placed between it and the eye, a final image (b a), 
situated within the focus of the biconvex lens. 

By this method we are able to detect the form of 
ametropia by the changes which take place in the size 
and shape of the optic disc, always remembering that 
the inverted image of the disc, produced by a convex 
lens at a certain fixed distance from the cornea, is 
larger in hypermetropia, and smaller in myopia, than 
in emmetropia. The lens should be held close to the 
patient's eye, and then gradually withdrawn, while the 
aerial image of the disc is steadily kept in view; the 
rapidity with which any increase or decrease takes 
place in the size of this image gives us an indication 
of the amount of the refractive error. 

If no change take place in the size of the image on 
thus withdrawing the objective the case is one of 
emmetropia, because the rays issue from such an eye 

Fig. 40. 




E. Emmetropic eye. Rays issuing parallel, image formed at 
the principal focus of the lens, no matter at what distance 
the lens is from the eye. 

parallel, and the image formed by the object-glass will 
always be situated at its principal focus, no matter 
at what distance the glass is from the observed eye 



INDIRECT EXAMINATION 



69 



(Fig. 40). As the distance of the image from the 
object-lens is always the same, the size of the image 
will also be the same. 

If diminution take place in the size of the image, 
the case is one of hypermetropia,, and the greater the 
diminution the higher is the hypermetropia. 

Fig. 41. 




Lens at 4 cm. from the cornea. 
Fig. 42. 




Lens at 12 cm. from the cornea. 

H. Hypermetropic eye. C. The centre of the lens. A B. Image 
on the retina, a b. Projected image. (3 a. The final image 
formed by the objective. 

This change in size may be explained by remem- 
bering that in hypermetropia the image of the disc 
formed by the object-glass is situated beyond its 



70 THE REFRACTION OF THE EYE 

principal focus, owing to the rays issuing from the 
eye being divergent; the relative size of the final 
image ]3 a to the object a b will therefore vary 
directly as the length c a, and inversely as the length 
c a ; so that on withdrawing the lens c from the ob- 
served eye, c a diminishes and c ft increases; there- 
fore the ratio of a ]3 to a b diminishes, i. e. the size 
of the image diminishes. The two diagrams Figs. 41 
and 42 show images formed by the object-glass 
when held at 4 cm. and at 12 cm. from the cornea, the 
latter image being the smaller. 

If the image become larger on withdrawing the 
object-glass, the case is one of myopia; the greater 
the increase of the image, the higher the myopia. 

This increase in the size of the image can also be 
explained with the help of mathematics, remembering 
that in myopia an inverted image is formed in front 
of the eye (Fig. 45), and it is of this we obtain an 
image with a convex glass placed between the eye 
and the inverted image, which we must regard as the 
object; the object and its image being both on the 
same side of the lens. 

In astigmatism, the disc, instead of appearing round, 
is frequently oval. If the image of the disc decrease 
in size in one meridian, while the other remain sta- 
tionary as the objective is withdrawn from the eye, 
it is a case of simple hypermetropic astigmatism. If 
the whole disc decrease in size, one meridian diminish- 
ing more than the other, it is compound hypermetropic 
astigmatism, the meridian being most hypermetropic 
which diminishes most. 



CONCAVE MIRROR AT A DISTANCE 71 

Increase in one meridian, the other remaining 
stationary, indicates simple myopic astigmatism. 

Increase in the size of the disc, but one meridian 
increasing more than the other, indicates compound 
myopic astigmatism, that meridian being most myopic 
which increases most. 

If one meridian increase while the other decrease, 
mixed astigmatism is our diagnosis. 

The Large Concave Mirror at a Distance. — If the 
observer be able to see the disc or some of the vessels 
with the mirror alone at a distance from the patient, 
the case is one of hypermetropia or myopia. The 
explanation of this is, that in emmetropia (Fig. 43) 
the rays which come from the two extremities of the 
disc (a b) emerge as two sets of parallel rays in the 

Fig. 43. 




same direction as the rays A c, b d, which, having 
passed through the nodal point, undergo no refraction. 
These two sets of rays soon diverge, leaving a space 
between them, so that an observer (unless he be quite 
close to the observed eye) is able only to bring rays 
from one point to a focus on his retina ; and there- 
fore, at a distance from the eye, the observer sees only 
a general illumination. 



72 THE REFRACTION OF THE EYE 

In hypermetropia (Fig. 44) the rays from the two 
points a b emerge from the eye in two sets of diverg- 
ing rays, in the same direction as the rays a c, b d, 
which undergo no refraction. These diverging rays 
have the appearance of coming from two points (a b) 
behind the eye, where an erect imaginary image is 
formed. 

Fig. J L. 




The more the rays diverge on exit, the sooner they 
will meet when prolonged backwards ; and hence the 
greater the hypermetropia, the Dearer will the image 
be to the nodal point. 

Fig. 15. 




The observer at a distance sees a clear, erect image 
which is formed behind the eye. 

In myopia (Fig. 45) the rays from the two points 



DIRECT EXAMINATION 73 

(a b) emerge as two converging sets of rays, which 
meet at a & on their secondary axes, thns forming an 
inverted image in front of the eye. This image can 
be distinctly seen by the observer if he be at a 
sufficient distance from the point, and accommodating 
for the particular spot at which the aerial image is 
formed. The higher the myopia, the nearer to the 
eye will this image be formed. 

From the above observations it will be understood 
that if the observer now move his head from side to 
side, and the vessels of the disc are seen to move in 
the same direction, the case will be one of hyperme- 
tropia, the image formed being an erect one. 

Should the vessels move in the opposite direction 
to the observer's head the case will be one of myopia, 
the image being an inverted one formed in the air in 
front of the eye. 

If the vessels of one meridian only are visible, then 
we have a case of astigmatism, hypermetropic if 
moving in the same, and myopic if moving in the 
opposite direction to the observer's head, that 
meridian being ametropic which is at right angles to 
the vessels seen. 

In mixed astigmatism the vessels of one meridian 
move against the observer's movements, and those of 
the other meridian with them : this is difficult to see. 

Thus we have obtained an indication of the form of 
ametropia. We may, however, estimate the amount 
of error by means of a refracting ophthalmoscope, of 
which there are many. 

The Direct Method. — By the direct examination we 



74 THE REFRACTION OF THE EYE 

obtain much more important information, not only of 
a qualitative, but also of a quantitative character. 

In endeavouring thus to estimate the refraction, it 
is essential that the accommodation of both the patient 
and observer be suspended. The observer first cor- 
rects any ametropia that he may have, either by 
having the proper correction in a suitable clip behind 
the sight-hole of his ophthalmoscope, or he may deduct 
his own ametropia from the glass which corrects the 
refraction of the patient and himself in the manner to 
be presently described. He then sits or stands as he 
may prefer on the same Bide as the eye he is about to 
examine, BO as to use his right eye for the patient's 
right, and his left for the patient's left. 

The light is placed on the side to be examined, a 
little behind and on a level with the patient's car; 
then with the mirror held close to the eye to be 
examined, so that the ophthalmoscope may occupy as 
nearly as possible the position of the spectacle glass, 
the observer looks for the disc. We really wish to 
estimate the refraction at the macula, but to this there 
are several obstacles : the light falling on this, the 
most sensitive part of the retina, has a very dazzling 
unpleasant effect for the patient, and causes the pupil 
to contract vigorously, the reflex from the cornea and 
the lens is exactly in the line of vision, and further 
there are no convenient vessels in this part which we 
may fix as test objects ; whereas the disc is but little 
sensible to light, and the vessels of this part, as well 
as the margins of the disc itself, are very convenient 
for our purpose, and although occasionally the refrac- 



DIRECT EXAMINATION 75 

tion of the macula and disc are not exactly the same, 
still practically it is sufficiently accurate to take that 
of the latter. 

To estimate the refraction by the direct method, 
it is necessary that the patient's accommodation 
should be relaxed; this will generally be the case 
when the examination is made in a dark room ; or a 
mydriatic may be used; then, if the observer's own 
accommodation be suspended, and the image of the 
disc appear quite clear and distinct, the case is one of 
emmetropia. This we know, because rays coining 
from an emmetropic eye (Fig. 46, e) in a state of repose 

Fig. 43. 




will issue parallel, and the observing eye receiving 
these rays will, if emmetropic with its accommodation 
suspended (which often requires great practice), be 
adapted for parallel rays, so that a clear image of a 
in the observed eye will be formed at b on the retina 
of the observing eye. 

Supposing the image does not appear clear and dis- 
tinct without an effort of accommodation, then we turn 
the wheel of the ophthalmoscope so as to bring for- 
ward convex glasses in front of the observing eye. 
The strongest positive glass with which we are able to 



76 THE REFRACTION OF THE EYE 

get a perfectly clear image of the disc is a measure of 
the hypermetropia, because rays comiug from a 
(Fig. 47) in the hypermetropic eye (h) issue in a 
divergent direction as though coming from e, the 
punctum remotum behind the eye. The convex lens 
l renders them parallel, and they then focus at h, on 
the retina of the observing emmetropic eye (e). 

Fig. 47. 




In practice many observers find it difficult or im- 
possible to tell if their own accommodation be com- 
pletely relaxed, so that if they see clearly the disc of 
the patient under examination, they do not at once 
assume that he is emmetropic, but only do so on find- 
ing that the weakest convex glass behind the ophthal- 
moscope impairs the clearness of the image. 

If, however, the image of the disc appear indistinct, 
and the convex glass, instead of rendering the image 
clearer, have the opposite effect, we must turn the 
wheel of the ophthalmoscope in the other direction, 
and so bring forward the concave glasses. The iceakest 
with which we can see the details of the fundus clearly 
is a measure of the myopia, because any stronger glass 
merely brings into play the accommodation of the 
observer. Rays from a (Fig. 48) leave the myopic 



D1EECT EXAMINATION 77 

eye (m) so convergent, that they would meet at (e) 
the punctum remotum. The concave lens l renders 
them parallel before falling on the relaxed eye (e) of 
the observer. 

Fig. 48. 




If the ophthalmoscope is not held very close to the 
eye, we must deduct from the focal distance of the 
lens the distance between the cornea and the in- 
strument in hyperinetropia, adding them together in 
myopia (p. 118). 

If astigmatism exist, the plan is to find the glass 
which enables the vertical vessels and lateral sides of 
the disc to be seen distinctly, and then the glass with 
which the vessels at right angles are best seen. 

Suppose the vertical vessels and lateral sides of the 
disc appear distinct without any glass, then the hori- 
zontal meridian, i. e. the meridian at right angles to 
the vessels clearly seen, is emmetropic ; and suppose 
also that the horizontal vessels with the upper and 
lower borders of the disc require a convex or con- 
cave glass to render them clear and distinct, then the 
vertical meridian is hypermetropic or myopic, and 
the case is one of simple hypermetropic or myopic 



astigmatism. 



If both the vertical and horizontal vessels can be 



78 THE REFRACTTON OF THE EYE 

seen with convex glasses, but with a stronger one for 
the vertical than for the horizontal, then the case is 
one of compound hypermetropic astigmatism, the 
horizontal meridian being the more hypermetropic. 

If both meridians had required concave glasses, 
but of different strengths, then the case would be one 
of compound myopic astigmatism. 

If the vertical vessels and the lateral sides of the 
disc can be seen distinctly through a convex glass, 
while the horizontal vessels require a concave glass, 
the case is one of mixed astigmatism, the horizontal 
meridian being hypermetropic, the vertical meridian 
myopic. 

The essential point to remember is, that the glass 
with which the vessels in one direction are seen is a 
measure of the refraction of the meridian at right 
angles to these vessels. 

The estimation of the refraction by the direct oph- 
thalmoscopic method is exceedingly valuable, but 
requires great practice; some observers find con- 
siderable difficulty in relaxing their accommodation 
completely, even after long practice. 

The student should take every opportunity to 
become thoroughly proficient in estimating the re- 
fraction by this method; in fact, every case that is 
not of an inflammatory character should be examined 
with the ophthalmoscope, and the refraction as esti- 
mated by the direct method recorded as a matter of 
routine; the ophthalmoscopic examination may con- 
veniently follow the testing of the patient's visual 
acuity. 



DIEECT EXAMINATION 79 

In hypermetropia and myopia one is able to esti- 
mate the amount of error accurately, and in cases 
of astigmatism where the chief meridians are hori- 
zontal and vertical one can come very near the exact 
correction, and without necessarily subjecting the 
patient to the inconvenience of a mydriatic : when 
the meridians are oblique the estimation is more 
difficult, because we may find no vessel whose course 
exactly corresponds with the chief meridians ; still 
the more this method is practised the more accurate 
will be the results obtained. The correction must 
always be confirmed by trying the patient at the test 
types with lenses, making any slight alteration that 
may be necessary. 

It is also an additional advantage that one can esti- 
mate the refraction at the same time that one makes 
an examination of the fundus. 

The comparison of the direct and indirect methods 
of examination is also very useful in astigmatism. If, 
for instance, the disc is elongated horizontally in the 
erect, and oval vertically in the inverted image, we 
know that the curvature of the cornea is greater in 
the horizontal than in the vertical meridian (see 
Figs. 86 and 87). 

The ametropic observer must always remember, 
when using the direct method for the estimation of 
errors of refraction, that he must correct his own 
defect either by wearing spectacles or by having a 
suitable glass in a clip behind his ophthalmoscope; 
he is then in the position of an emmetrope ; but, if 
he prefer it, he may subtract the amount of his own 



80 THE EEFRACriON OE THE EYE 

hyperrnetropia or rnyopia from trie glass with which 
lie sees clearly tlie patient's discs. Thus, if the ob- 

- i ~er have 2 D. of hypercnetropia, and require + 3 D. 

- see thefnndus clearly, [ +3 D. - (+ 2D.) = + 1 D., 

the patient would have 1 D. of hypernietropia; had he 
required — 2 D., then the observer would have 4 D. 
of myopia, because (-2D. - +2D.)=-4D. 

The same with the myopic observer : if his myopia 
amount to 3 D., then he will require — 3D.:: sec 
clearly the emmetropic fundus ; if he see the disc well 
without a glass, then the eye under examination has 
3 D. of hypermetropia : if he require a -f 2 D., then 
the hyperraetropia will be 5 D., and so on. 

Ametropia may also be easily recognised in the 
following" manner : — The fundus being illuminated by 
a mirror about one metre from the patient, if the eye be 
emmetropic the rays of light will return parallel to one 
another, and a red reflex can only be obtained when 
the observing eye is in the path of these rays, that is 
behind the perforation of the mirror. If hyperme- 
tropic the returning rays will diver g _ and 

Fig. 49. 




the observer will notice, as he moves his eye (b) from 
behind the mirror at L (and at right angles to the 



WITH THE MIRE0E 



81 



visual axis of the patient, who should fix on the centre 
of the mirror), that the last ray of light (a h') is seen, 
or, in other words, the red reflex disappears, on the 
same side of the pupil as that to which the observer 
moves his head. 

If the eye be myopic the rays will converge, 
cross, and diverge (Fig. 50) ; when the error is 1 D. 

Fig. 50. 




or more, the last ray of light is seen, or the red reflex 
disappears, on the opposite side of the pupil. A single 
trial of this will prove its correctness. 

The endeavour to estimate the amount of myopia 
or hypermetropia by measuring the distance between 
the perforation of the mirror, and the point at which 
the last ray was seen, has been unsuccessful, owing to 
the varying size of the pupil. 

The ophthalmometer of Javal and Schiotz, and 
Tweedy' s optometer, can, I think, be more conveniently 
considered when treating* of astismiatism. 



82 THE REFRACTION OF THE EYE 



CHAPTER V 



Rettsoscopy, or the shadow test, is deservedly one 
of the most popular objective methods of estimating 
the refraction of the eye. 

It has the great advantage of being easily learnt, 
and can be carried out quickly, saving much time in 
difficult cases of astigmatism; it is especially useful 
in young children, in amblyopic patients, and in 
malingerers ; besides, it enables very small degrees of 
astigmatism to be detected which but for this method 
would probably escape notice. Ketinoscopy may be 
carried out either with a concave or a plane mirror. 

Rhinoscopy with the- Cor.: Mirror. 

If light from the ophthalmoscope lamp be reflected 
into the eye by means of a concave mirror, at a dis- 
tance of a metre or so, an observer looking through 
the sight-hole of the mirror will notice the ordinary 
red fundus reflex ; on slightly rotating the mirror the 
iUuminated area of the pupil may disappear (or, what 
may be more easily seen, the edge of the shadow 
bounding this illuminated area may appear), on the 
sam the rotation or in the opposite direction, 



EETINOSCOPY 83 

according to the refraction of the eye under observa- 
tion : thus if the mirror be rotated to the right, and 
the edge of the shadow move across the pupil also to 
the right, i. e. in the same direction as the rotation of 
the mirror, the case is one of myopia; whereas if the 
shadow had moved in the opposite direction to the 
mirror, the case would be one of hypermetropia. 

This method of employing retinoscopy is so simple 
that a few practical trials will suffice to make it 
understood, although, of course, as in all other mani- 
pulations some little practice is required in giving to 
the mirror the necessary movements, and enabling 
one to appreciate what is seen. 

A mydriatic is not absolutely essential ; still, when 
we have to examine young people and children, the 
use of atropine is most certainly advisable. In the 
first place, the dilatation of the pupil renders our 
examination so much easier ; and secondly, atropine 
enables us to arrive at a more accurate estimation 
by thoroughly paralysing the accommodation : for 
although the examination take place in a dark room, 
and with the patient looking into distance, it must be 
remembered that there is often (especially in the 
case of children) some accommodation still in force : 
or there may be spasm of the ciliary muscle. 

In persons over the age of twenty, atropine is not 
usually employed owing to the great discomfort 
entailed by its paralysing effects on the ciliary muscles, 
which lasts for seven or eight days, and because the 
use of this drug has occasionally produced glaucoma 
in people beyond middle life ; we may, however, wish 



THE EEFEACTIOX OF THE ITI 

to dilate the pupil? with a mydriatic that acts fully 

and quickly, and whose effects last "but a short time. 

The most conyenient combination for this pur]:' - . ; - — 

R Homatropinse Hydrobronuitis, gT. it; 
i nse Hydrockloratis, gr. x; 

Acidi Salicylici, gr. j ; 

Aq. Destillatse, 3J. 
7: gatibe. 

A drop or two of this solution applied two or three 
times at interrals of ten minutes produces rapidly a 
maximum dilatatiun of the pupil, which passes off in 
about twelye hours. 

Another great adyantage of a mydriatic is that the 
refraction of the macula can be measured, whereas 
when the pupil is not dilated we haye to be satisfied 
with the refraction at the optic discs, which may 
occasionally yary considerably from that found at the 
macula : the estimation of the refraction at the macula 
constitutes one of the chief adyantages that re tin o- 
scopy possesses oyer the measurement obtained by 
the direct method. 

7 : examine the patient, then, we dilate his pupils, 
and seat him in a dark room, with a lamp placed oyer 
his head, so far back that it throws no direct rays 
upon his face ; the lamp in this position will not 
require moying during the examination of either 
The obseryer takes up a position about 120 cm. in 
front of the patient, and directing him to look at the 
perforation in the concaye mirror (which should haye 
a focus of 25 cm.), the obseryer will be enabled to 
reflect the light along the yisual axis, and thus obtain 
the ordinary red fundus reflex. 



KETIN0SC0PY 85 

If a mydriatic has not been used, this procedure 
will cause the pupil so to contract, that it will be 
difficult to see the shadows ; and in that case the ob- 
server must make the patient look a little inwards, so 
that the light may be reflected along the optic axis. 
If we now rotate the mirror slightly from side to side 
on its vertical axis, we see a shadow come out from 
behind the pupil, moving horizontally across the 
illuminated part. The edge of this shadow may be 
linear or somewhat crescentic : its direction may vary, 
being either vertical or oblique ; if oblique, then we 
know at once that astigmatism is present. The shadow 
moves either in the same or the opposite direction to 
the mirror, so that when the latter is tilted to the 
right the shadow may come from the left, or vice versa. 

Thus, assuming the shadow's edge to be vertical 
(Fig. 51), if it move with the mirror the case is one 
of myopia ; but if it move against or in the opposite 
direction to the mirror, the case is either one of hyper- 
metropia, emmetropia, or low myopia. 

Fig. 51. 





What is this shadow whose edge we see ? How 
and where is it formed ? and what influences its 
movements and clearness ? 



86 THE REFRACTION OF THE EYE 

To enable us to answer these questions, we will 
place before a screen a convex lens, at such a distance 

Fig. 52. 




a. The concave mirror, h. Tiie candle, c. The lens. d. Small 
image of candle formed on screen, e. The screen, f. Dark 
shadow aronnd. 

from it that converging rays from a concave mi: 
having crossed and become divergent, are brought to 
an exact focus, and there is then formed a very small, 
erect, well-defined image on the screen of the lamp 

Fig. 53. 




At fa small image of the candle is formed ; at d and/, circles 
of diffusion. 

from which the concave mirror received its r 
erect, because it has suffered two inversions. 



KETINOSCOPY 87 

This image of the lamp is surrounded by a sharply 
defined and dense shadow. 

If we move the lens nearer to, or farther from the 
screen, the area of light becomes larger, and the illu- 
mination feebler. 

The mirror being rotated on its vertical axis, the 
image of the candle, with the surrounding shadow, 
will always be found to move in the opposite direction 
to the mirror, whatever be the distance of the lens 
from the screen. 

This is exactly what takes place in the eye, of 
which our screen and lens are a representation. 

It may therefore be stated that the illumination 
and shadows which we see are an enlarged image of 
the lamp with the surrounding shadow, brought more 
or less to a focus on the retina according to the re- 
fraction of the eye. They always move against the 

Fig. 54. 




31. The mirror, si/. The mirror after rotation. The ex- 
tremities of the dotted line have moved in the opposite 
direction to the rotation of the mirror. 

mirror : but as these movements are seen through the 
transparent media of the eye, and thereby undergo 
refraction, the " apparent" may differ from the "real" 



88 THE KEFEACTIOX OF THE KYI 

movements. The image A the lamp, and its 

surrounding shadow, are formed in the same manner 
D other images ; and it may be well to repeat 
here what has already been said with regard to the 
formation of these imac: e 

In emmetropia the image is formed at infinity ; thus 

Fie. 




the rays which come from the two extremities, a, e, 
emerge as fcwi - - £ parallel rays, in the same direc- 
tion as the rays a c. e d ; which, having passed through 
the nodal point, undergo no refraction. These two 
of rays soon diverge, leaving a space between 
them, so that an observer, un] be quite close to 

the observed eye, is unable to bring these rays to a 
focus on his retina; and, therefore, at a distance from 
the eye the observer sees only a diffused and blurred 
ima^ 

In hvpermetropia the image is formed behind the 
: thus the rays from the two points a, b (Fig. 56) 
emerge from the eye in fcw< sets t diverging rays, 
in the same direction as the rays a c, b d, which un- 
dergo no refraction. These diverging rays have the 
appearance of coming from two points a, L, behind 
the eye, where an erect irnag led, a b. 



ItETINOSCOPY 



89 



The more the rajs diverge on exit, tlie sooner they 
will meet when prolonged backwards ; and hence the 



Fig. 56. 




greater the hypermetropia, the nearer will the image 
be to the nodal point. 

The observer, at a distance, sees a clear, erect 
image, which is formed behind the eye. 

In myopia an inverted image is formed in the air 
in front of the eye ; thus the rays from the two points 
a, B, emerge as two converging sets of rays, which 

Fig. 57. 




meet at a, h, on their secondary axes, thus forming an 
inverted image in front of the eye. This image can 
be distinctly seen by the observer if he be at a suffi- 
cient distance from the point, and accommodating 



90 THE REFRACTION OF THE EYE 

for the particular spot at which the aerial image is 
formed. 

We have already seen that the real movements of 
the shadows on the retina are against the mirror. 

In hypermetropia the final image of the candle and 
its surrounding shadow, produced by the concave 
mirror, is aD erect one formed behind the eye, and as 
it is viewed through the dioptric system of the eye, it 
therefore moves against the mirror. 

In myopia the final image is an inverted one, pro- 
jected forwards. This, therefore, moves with the 
mirror, it having undergone one more inversion. 

To this rule, that in myopia the image moves with 
the concave mirror, there are two exceptions : 

1st. If the observer be nearer the patient than his 
far point, but not within the focal distance of the 
mirror, the imasre will move against the mirror. This 

J CD O 

is frequently the case in low degrees of myopia, where 
the patient's far point is beyond 120 cm. 

2nd. If the observer be within the focal distance of 
the mirror, although beyond the far point of the 
patient, the image will in this case also move against 
the mirror. This source of error can always be 
avoided by using a concave mirror of 25 cm. focus, 
and keeping 120 cm. from the patient. 

Therefore, if the image move with the mirror, the 
case is certainly one of myopia. If it move against 
the mirror, it is most likely one of hypermetropia ; 
but it may be emmetropia, or a very low degree of 
myopia. 

The movements tell us the form of ametropia we 



EETINOSCOPY 91 

have to deal with. The extent of the movements on 
rotation of the mirror, the clearness of the image, and 
the brightness of its edge, enable us to judge approxi- 
mately the amount of ametropia to be corrected; 
some practice, however, is required before we can 
form an opinion with anything like accuracy. 

The extent and rate of movement is always in 
inverse proportion to the ametropia ; the greater the 
error of refraction, the less the movement, and the 
slower does it take place. This may be explained in 
the following way : 

Suppose A to be the image of a luminous point 
formed on the retina, and that a line be drawn from A 
through the nodal point b to c. Now, if the case be 
one of myopia (Fig. 58), an inverted projected image 
of A is formed somewhere on this line, say at c. The 
higher the myopia, the nearer to the nodal point will 
this image be ; and hence we may suppose it formed 
as near as d. If the mirror be now rotated, so that it 

Fig. 58. 




take up the position of the dotted line m', c will have 
moved to c, and d to d ; hence it is clear that c has 
made a greater movement than d. 



92 THE REFRACTION OF THE EYE 

Had the case been one of hypermetropia (Fig. 59), 
the image would have been projected backwards, and, 

Fig. 59. 




as in myopia, the higher the degree of error the 
nearer to the nodal point is the image formed. 

In this cast- tin 1 line from the nodal point b to A is 
prolonged backwards, and the image of the luminous 
point in a low degree of hypermetropia is formed say 
at c, and in a higher degree say at d. On moving 
the mirror into the position of the dotted line m', c 
moves to c, and d to d ; hence it is clear that c has 
made a greater movement than d. 

Therefore, as the ametropia increases, the extent 
of the movement of the image decreases. The clear- 
ness of the image and the brightness of its edge 
decrease as the ametropia increases. 

It was shown in Fig. 53 that on placing before 
a screen a convex lens, at such a distance that con- 
verging rays from a concave mirror, having* crossed 
and become divergent, are brought to an exact 
focus, forming a small erect, well-defined image on 
the screen of the lamp from which the concave 
mirror received its rays, on moving the lens nearer 



RETINOSCOPY 93 

to or farther from the screen, the larger becomes the 
area of light, and the feebler the illumination, owing 
to the circle of diffusion formed on the screen. 

Therefore, in the case of the eye, the greater the 
ametropia, the larger is the circle of diffusion and the 
weaker the illumination, so that the image we see is 
less bright and its edge less distinct. 

It is, therefore, in the lower degrees of ametropia 
that the brightest and best defined shadows are seen. 

Having thus answered the questions concerning the 
shadows which we see in retinoscopy, we are in a 
position to pursue further the practical working of 
the subject, with special reference to the correction of 
any existing error of refraction by glasses. 

The patient, then, being seated in the dark room, 
the pupils dilated, and the lamp over his head, as 
before described, we take up our position 120 cm. 
in front, with a concave mirror of 25 cm. focus (a 
(xalezowski mirror is the one commonly used, and is 
found convenient). The patient is then directed to 
look at the centre of the mirror, so that the light 
from the lamp may be reflected along the visual axis. 
On looking through the perforation of the mirror we 
get the ordinary fundus reflex, bright if the patient 
be emmetropic, less so if he be ametropic; and the 
greater the ametropia, the less bright will the fundus 
reflex be. We now rotate the mirror on its vertical 
axis to the right; if a vertical shadow come across 
the pupil from the patient's right, i. e. in the same 
direction as the movement of the mirror, or, what is 
the same thing, if the shadow move in the same 



92 



THE REFRACTION OF THE EYE 



direction as the circle of light on the patient's face, 
the case is one of myopia. Should the edge of the 
image appear well denned and move quickly, in addi- 
tion to a bright fundus reflex, we infer that the 
myopia is of low degree, and proceed to correct it. 

Each eye must, of course, be tried separately. 

The patient having put on a pair of trial spectacle- 
frames, we place a weak concave glass, say — 1 D., 
before the eye we arc about to correct. If the image 
still move with the mirror, we place in the frame 
— 1*5 D., then — 2 D., and so on, until we find the 
point at which no distinct shadow can be seen. 
Supposing this to be — 2 D., and that on trying — 2*5 
D. the image move against the mirror, — 2 D. is 
assumed to be the correcting glass. This, however, 
will be found not to be the full correction of the 
myopia, because the observer being 120 cm. away, 
when tin- patient's fat- point approaches that distance 
the observer will be unable to distinguish the move- 
ments of the shadow ; and when the far point of the 
observed, though not situated at infinity, is still at a 
greater distance than the observer, the shadow moves 
in the opposite direction. Hence it is customary in 
cases of myopia to add on — 1 D. to the correcting 
lens, and this would give us — 3 D. as the proper 
glass for our case. 

In correcting myopia it is a convenient and reliable 
plan to stop at the weakest concave glass which makes 
the image move against the mirror, and put that 
down as the correcting lens. 

When the myopia is of high degree, and a strong 



RETINOSCOPY 95 

concave glass has to be used for its correction, the 
light reflected from the mirror is so spread out by the 
concave glass that fewer rays pass into the eye, and 
therefore the illumination is not so good as in other 
states of refraction, and the examination becomes 
more difficult. 

Had we obtained a reverse shadow we should then 
try convex glasses, when if + "5D. neutralised it, we 
should assume the case to have been one of low 
myopia. Had it required + 1 D., then it would be 
one of emmetropia ; above this, hypermetropia. We 
proceed exactly as before, putting up stronger and 
stronger lenses, until we are unable to make out the 
movements of the image. This is assumed to be the 
correcting glass; and just as in the above case the 
myopia was under-corrected, so in this the hyperme- 
tropia is slightly over-corrected, and hence it is usual 
to deduct from this glass + ID.; or we may stop at 
the strongest convex glass with which we still get a 
reverse shadow. 

To sum up, therefore, if the shadow move with the 
mirror, it is a case of " myopia ;" if against, it may be 
weak myopia if + *5 D. cause the image to move with 
the mirror; emmetropia if + 1 D. neutralises it; 
hypermetropia if a stronger glass is required. 

The points to be observed are — (1) the direction of 
the movement of the image, as indicating the kind of 
ametropia : (2) the rate and amount of movement, (3) 
the brightness of the edge of the image, and (4) the 
amount of fundus reflex all indicate the degree of 
ametropia. 



96 THE REFRACTION OF THE EYE 

We have taken notice only of the horizontal axis, 
but any other meridian will, of course, do equally 
well, if the case be one of hypermetropia or myopia 
simply. If, however, the case be one of astigmatism, 
then the refraction of the two chief meridians will 
differ. 

In astigmatism, the image of the flame of the candle 
formed on the retina is distorted so as to be more or 
less of an oval form, according to the position of the 
retina and the maximum and minimum curvatures of 
the cornea. 

In the normal eye the focus of the vertical meridian 
of the cornea is slightly shorter than that of the hori- 
zontal. So long as no impairment of vision occurs, 
the eye is said to be normal, and we speak of the 
condition as normal regular astigmatism. TYhen, 
however, the acuteness of vision is diminished, then 
astigmatism is said to exist. 

Parallel rays, passing through a convex spherical 
lens (disregarding some slight irregularities due to 
aberration), form a cone, any section of which, per- 
pendicular to its axis, will be. a circle. The size of 
the circle depends upon the distance of the point at 
which the lens is from its focus. If the cone be di- 
vided beyond the focus as in myopia, the rays having 
crossed and become divergent, a circle of diffusion is 
formed on the retina. In hypermetropia the cone is 
divided before having come to a focus, and thus 
forms a diffusion circle. But in astigmatism the 
divided cone is circular only at one point, No. 4 
in Figs. 76 and 77. To explain tin's, we place in 



RETIN0SC0PY 97 

front of the convex spherical glass, a weak convex 
cylindrical glass, with its axis horizontal. The result 
of this is, that parallel rays passing through this 
combination do not form a circular cone, because the 
rays which pass through the vertical meridian, come 
to a focus before those passing through the horizontal, 
as shown in Fig. 76. 

The rays being divided at 1, an oblate oval is formed; 
at 2, a horizontal straight line, the vertical rays 
having come to a focus ; at 3, 4, 5, the vertical rays 
have crossed and are diverging-, and the horizontal 
rays are approaching ; at 4, a circle is formed ; at 6, 
a vertical straight line, the horizontal rays having met 
while the vertical are still diverging ; a large prolate 
ellipse is formed at 7. 

So that, in astigmatism, the image on the retina may 
be more or less of an oval; instead of being either a 
small well-defined image of the candle, or a circle of 
diffusion, as in the case of emmetropia, myopia, or 
hypermetropia. This oval may have its edges hori- 
zontal and vertical; frequently, however, they are 
more or less oblique, as shown in the following 
figures (Fig. 60). 

The oblique movements of the shadows are inde- 
pendent of the direction in which the mirror is 
rotated. 

This obliquity is produced thus (Fig. 61) : — if we 
cut a circular opening in a piece of cardboard to 
represent the pupil, and then place behind it an oval 
piece of card which is to represent the shadow, so that 
that part of its edge which occupies the pupil has an 

7 



98 



THE REFRACTION OF THE EYE 



oblique position, then on moving the card across in 
the direction o i», it has the appearance of moving in 



Fig. 00. 




liqae shadows in astigmatism. 

the direction o < . at right angles to the edge of the 
card. Hence the direction of the shadow's movement 
is deceiving, and it- oblique edge is due to the facl 
thai only that edge which coincides in direction with 
one of the principal meridians is Been well defined by 
the observer. Therefore the apparent movements 
correspond with the edge of the shadow. 

Fig. 61. 




The same takes place in astigmatism, the two chief 
meridians of which are parallel and perpendicular to 

In retinoscopVj therefore, 



the edge of the shadow. 



RETIN0SC0PY 99 

when the edge of the image is oblique, we know at 
once that the case is one of astigmatism. 

Another characteristic appearance that will be 
sometimes met with in astigmatism, is, that the fundus 
illumination may assume a band-like shape something 
like Fig. 62; and on tilting the mirror on an axis 
parallel to this band, a dark shadow will appear to 

Fig. 62. Fig. 63. 





come from both edges of the pupil at once, uniting in 
the centre to form a black band, leaving the upper and 
lower part of the fundus illuminated as shown in 
Fig. 63. This band is parallel with one of the chief 
meridians, and indicates the point of exact neutralisa- 
tion of the meridian at right angles to it. The effect 
is due to the retinal images being now in the form of 
a line (see Figs. 76 and 77, ii and vi) . 

Supposing we take a case in which the meridians 
are horizontal and vertical, we judge if one shadow 
be more distinct or quicker in its movements than the 
other, though it is not always easy to recognise the 
presence of astigmatism at once, so that it is necessary 
to commence to correct one meridian. If the shadow 
move against in all meridians, we first take the vertical, 
and put up in front of the patient, in the spectacle-frame, 



100 THE EE FRACTION OF THE EYE 

convex spherical glasses, until we find the strongest 
with which the shadow still inoves against the mirror. 
We put this down as the correcting-glass for the 
vertical meridian, and let us suppose that glass to be 
+ 2 D. TTe next take notice of the horizontal 
meridian, and if -f 2 D. is also the highest glass with 
which we still get a reverse shadow, then of course 
we know the case is one of simple hypermetropia. 
But supposing the highest convex glass had been 
-4- 4 D., we indicate it conveniently thus : 

+ 2 D. 

-t4D. 

The case is one of compound hypermetropic astigma- 
tism, and should require for its correction | 2 D. 
sphere combined with -f 2 D. cylinder axis vertical. 

We will take another case — that in which the verti- 
cal meridian requires — 2 D. to give a reverse shadow, 
while in the horizontal meridian -f 2 D. is found to be 
the highest convex glass with which we still obtain 
a reverse shadow. Here we have a case of mixed 
astigmatism, which can be corrected in either of the 
three following ways : 

1st. —2D. cylinder axis horizontal combined with 
+ 2 D. cylinder axis vertical ; this is a plan seldom 
used, and is not so easy to work with as a sphere and 
a cylinder. 

2nd. —2D. sphere combined with +4D. cylinder 
axis vertical, or 

3rd. + 2 D. sphere combined with — 4 D. cylinder 
axi> horizontal* This last is perhaps the preferable 



EETINOSCOPY 101 

plan. Opticians like working — cylinders on to + 
spheres, rather than + cylinders npon — spheres. 

Supposing the axis of the shadow to be oblique 
(Fig. 60), we know at once that astigmatism exists, 
and we proceed to correct each meridian separately, 
moving the mirror at right angles to the edge of the 
shadow, not horizontally and vertically. We judge of 
the amount of obliquity by the eye, and can frequently 
tell within a few degrees. If the vertical meridian be 
20° out, and require for its correction — 2 D., and 
the axis at right angles to this (which will therefore 
be at 110°) require — 3 D., we express it as Fig. 64, 
and correct it with sphere — 2D. combined with 
cylinder — ID. axis 20° : this case is one of compound 
myopic astigmatism. 

Often one is able to put up the cylinder in the 
spectacle-frame with the exact degree of obliquity. 

Fig. 64. 




Having found the glasses which correct the two 
meridians, we put up the combination in a spectacle 
trial frame, and if we now get only a slightly reversed 
shadow in every direction, the glasses are assumed to 
be the right ones, and we proceed to confirm it by 



102 THE REFRACTION OF THE EYE 

trying the patient at the distant type, making any 
slight alterations that may be necessary. 

I cannot too strongly recommend the use of sulphate 
of atropine in solution, gr. iv to jj, frequently dropped 
into the eyes for three days prior to the examination, 
so as thoroughly to relax the accommodation. It can 
be used without fear, and without a great amount of 
inconvenience in most young people under twenty 
years of age. I have worked out a great many cases 
of astigmatism, and feel more and more the necessity 
of using this drug to enable one to arrive at exact 
results. I might say that 1 have seldom seen a 
young person whose astigmatism has been worked out 
without atropine wearing the right correction; and 
the inconvenience entailed upon the patient for two 
weeks by its use is not to be compared to the trouble 
and asthenopia from which he is so liable to suffer if 
the glasses worn are not the proper ones. 

Fetinoscopy with the Plane Mirror. 

Many observers prefer to use the plane mirror, they 
consider that the shadows are more easily seen, and 
that the results obtained are more exact than those 
found with the concave mirror. I now use the plane 
mirror entirely, and find it works well ; much depends, 
however, upon practice and the mirror one is accus- 
tomed to. 

The essential point to remember is that the move- 
ments of the shadows with the plane mirror are the 
reverse of those which have been described under the 
concave mirror, viz. the shadows move in the same 



RET1N0SC0PY 103 

direction as the movement of the mirror in hyperme- 
tropia, and in the opposite direction in myopia. 

The explanation is that the concave mirror pro- 
duces a real image of the light in front of it, be- 
tween the mirror and the patient; this image becomes 
the object for the observed eye, and therefore the 
light on the retina moves in the opposite direction to 
the mirror. The image from the plane mirror is a 
virtual one situated behind the mirror, so that the 
real movement of the light on the retina is with the 
mirror. 

The following diagram (Fig. 65) may help to make 
this clear. 

Fig. 65. 




Eays of light from L fall on the plane mirror a, and 
are reflected as divergent rays into the eye as if 
coming from point G behind the mirror; these rays 
focus on the retina at c. On tilting the mirror into 
position b, the rays from l diverge from the mirror (as 
if coming from h) and focus at d ; therefore the real 
movement of the light on the retina is with the mirror. 



104 THE REFRACTION OF THE EYE 

Dr. Jackson of Philadelphia has proposed a farther 
modification; he uses a plane mirror, and thus de- 
scribes the practical application of the method. 

"Simple myopia. — Eays of light from any given 
point of the retina emerge from the myopic eye con- 
vergent, and meet at the point in front of the eye for 
which the eye is optically adjusted. The accommo- 
dation being in abeyance, this will be the far point of 
distinct vision. So that there is formed at the far 
point of the myopic eye an inverted image of the 
retina. If now the eye of the observer be placed 
between the patient's eye and its far point, there will 
be seen an erect image of the patient's retina ; but if 
the observer view the patient's eye from somewhere 
beyond its far point, he will see not an erect image, 
but the inverted image formed at the far point. In 
the first case the boundary of light and shade which 
marks the border of the retinal area will appear to 
move with the facial area ; in the second case, against 
it. In practice the surgeon begins the examination 
somewhat more distant from the patient than the far 
point of the eye under examination. Then he slowly 
approaches the patient, all the while watching the 
apparent movement of the retinal area produced by 
slightly rotating the mirror from side to side about its 
axis. 

"As long as this apparent movement is opposed to 
that of the facial area, the surgeon knows he is watch- 
ing the inverted image beyond the patient's far point. 
Presently, however, the direction of the movement of 
the retinal area cannot be distinguished, the far point 



RETINOSCOPY 105 

has now been reached ; and coming still closer the 
apparent movement again becomes distinct, but is 
seen to correspond in direction with the real move- 
ment; the far point has now been passed, and the 
patient's retina is being viewed in the erect image. 
By noting the point at which this reversal occurs, 
the surgeon notes the far point of the eye under ob- 
servation ; by measuring the distance from this point 
of reversal to the eye, he measures the distance from 
the patient to his far point of distinct vision ; and 
the reciprocal of this distance, of course, expresses 
the degree of his myopia. Thus, supposing the point 
of reversal to be one fourth of a metre in front of the 
eye, one divided by one fourth equals four, the 
number of dioptres of myopia present. 

" Theoretically, the method as now described is 
complete, but for convenience and accuracy in its 
application, one or two other points must be attended 
to. When the observer's eye has come quite close to 
the patient's, say to within one eighth of a metre, 
and the inverted image is still seen between them, it 
is best to place a concave lens ( — 8 D.) before the 
patient's eye, and then to estimate the amount of 
myopia remaining uncorrected; and by adding it to 
the amount which the lens used has corrected, deter- 
mining the total myopia present. When the observer 
has approached so near the inverted image that it lies 
closer to his eye than his near point of distinct vision, 
he can no longer see that image distinctly. Still he 
can distinguish in which direction the retinal area 
appears to move, until he approaches somewhat 



106 THE REFRACTION OF THE EYE 

nearer to the image, when the circles of diffusion 
upon his own retina become so large that the retinal 
area of light, seen in the patient's pupil, seems very 
diffuse and faint, and the direction of its apparent 
movement uncertain. Because of this there is great 
practical difficulty in determining exactly where the 
point of reversal is situated. Now it is evident that 
if the point of reversal is within a few inches of the 
eye, an error of two or three inches as to its position 
entails an error of some dioptres in the amount of 
myopia present. Therefore, when by the method 
above described the degree of myopia has been 
approximately ascertained, place before the patient's 
eye ;i concave lens Btrong enough to remove the point 
of reversal a metre or more from the eye. At such 
a distance, an error of two or three inches as to the 
position of the point of reversal is of no consequence; 
and an accurate determination of the remaining, and 
hence of the total myopia can readily be made. 
Having determined the amount of myopia present, 
the surgeon will of course be guided by the rules he 
would follow had the myopia been measured by any 
other method. 

(t Ryperinetropia. — On viewing the fundus reflex it 
is found that at all distances the erect image is seen, 
and the retinal area appears to move with the facial 
area. Place before the patient's eye a convex lens 
strong enough to over-correct the hypermetropia. 
Then, by the method given above, determine the de- 
gree of myopia so produced. Deduct this amount of 
myopia from the strength of the convex lens used; 



RETINOSCOPY 107 

and the remainder will express the degree of hyper- 
metropia present. Suppose, for example, the hyper- 
metropia amounts to four dioptres. Placing a five- 
dioptre convex lens before the eye, it is found that 
one dioptre of myopia is produced, the point of reversal 
being at one metre. Then five, minus one, equals four, 
which expresses in dioptres the amount of hyper- 
metropia present. Should it be found that the + 5D. 
lens leaves the eye hypermetropic, so that the erect 
image is seen at all distances, replace it by a + 10 D., 
and proceed as before. As in myopia, however, the 
final accurate determination should be made at a dis- 
tance of not less than one metre. It may be noticed 
that low degrees of myopia may be measured without 
the use of any lens, but that to determine the degree 
of hypermetropia present a convex lens is always 
necessary. 

" Emmetrojpia is determined by the method for 
measuring hypermetropia. The convex lens being 
placed before the eye, the resulting myopia is found 
to equal exactly the strength of the lens in use. 

"Regular astigmatism. — In applying the test to 
the measurement of regular astigmatism, instead of 
rotating the mirror about any axis, vertical, horizontal, 
or oblique, as may be done when the curvature of the 
cornea is the same in all directions, it is rotated 
about axes perpendicular to the directions of the 
principal meridians of curvature, and the point of 
reversal thus found for each principal meridian. To 
determine the direction of these principal meridians, 
the eye, if not previously so, should be rendered 



108 THE REFRACTION OF THE EYE 

myopic in all meridians, and then viewed from different 
distances. It will then be found that at certain points 
the fundus reflex takes the shape of a more or less 
distinct band of light stretching across the pupil, 
while on one or both sides of it may be seen a shaded 
area, the f somewhat linear shadow' of Bowman. 
This band of light is very readily moved in a direction 
perpendicular to its length, but in the direction of its 
length cannot be made to move at all. The point 
where this appearance is presented is the point of 
reversal for that principal meridian of the cornea 
wli<>>(.> direction coincides with the length of the band. 
The other principal meridian is, of course, at right 
angles to this, and the observer, by placing his eye 
at its point of reversal, will be in position to see a 
similar band extending in a direction perpendicular 
to that <>t the band first observed. This use of the 
shadow test may be made clearer by the consideration 
of what occurs in a particular case. Suppose the 
patient's cornea to have such a curvature as to 
cause in the horizontal meridian (axis vertical) a 
hvpermetropia of four dioptres, and in the vertical 
meridian (axis horizontal) a myopia of one dioptre. 
Place before the eye a + 5 D. spherical lens. On 
approaching it from a distance, it is found that the 
retinal area moves against the facial area in all 
directions. But as the distance of one metre is 
approached, it is noticed that the retinal area takes 
the form of a horizontal band, readily moveable 
upward or downward, but difficult to move to the 
right or left j and when the point of one metre is 



EET1N0SC0PY 109 

reached, all movement to the right or left ceases, 
and the band is more distinct. Going still closer, 
the point of reversal for the horizontal meridian 
being passed, movement to the right or left re- 
appears, but it is now with the facial area. The 
movement upward or downward is still against 
that of the facial area. As the patient is still ap- 
proached, the appearance of a horizontal band fades 
out, and presently is replaced by that of a vertical 
band. The vertical band moves readily to the right 
or left, but less distinctly upward or downward, and 
at one sixth of a metre all vertical motion is lost. 
This is the point of reversal for the vertical meridian. 
On approaching still closer, vertical movement re- 
appears, but like the horizontal movement it is now 
with the facial area, not against it. Thus it is found 
that for the horizontal meridian the point of reversal 
is one metre distant from the eye, and that for the 
vertical meridian the point of reversal is one sixth 
metre distant. That is, the use of the convex lens 
has made the eye myopic in the one meridian one 
dioptre, in the other meridian six dioptres; and by 
taking into account the effect of the spherical lens 
used, the mixed astigmatism is seen to be what we 
supposed it. But for accurate work, as in simple 
myopia and hypermetropia, the degree of ametropia 
for each meridian should be finally determined with 
such a lens before the eye as would place the point of 
reversal, for that meridian, one metre or more dis- 
tant." 

A few cases from my note-book will do more than 



110 THE REFRACTION OF THE EYE 

any description to elucidate the subject of retinoscopy 

as carried out with the concave mirror. 

Case 1. Spasm of Ciliary Muscle. — Boy aged 11 

years. 

r.V. t <v-i D.=f. 

Bright fundus reflex, shadow moves with the mirror, 
but with — # 5 D. a reverse shadow is seen. The case, 
therefore, looks like one of weak myopia. Ordered 
guttse atropiae, gr. iv to ^j three times a day ; on the 
third day, with retinoscopy, + 1*5 D. still gives an 
opposite shadow. On trying the patient at the distant 
type with + 1 *5 D. both eyes read -| well. This, there- 
fore, was a case of hypermetropia simulating weak 
myopia, due to ciliary spasm : such cases are not rare. 

Case 2. Hypermetropia. — Girl aged 13, suffering 
from " tinea tarsi." 

R.V.f Hm.l D. = f. 
L.V.f Hm.ll).=|. 

Guttae atropiae sulphatis, gr. iv to gj. Fundus reflex 
moderate ; a reverse shadow is seen moving somewhat 
slowly. On trying + 2 D., shadows become much 
more distinct and the movement quicker; -f 4 D. is 
found to be the strongest glass with which we still 
get a reverse shadow. With + 4 D. | was read, but 
with no stronger glass ; this, therefore, is the measure 
of the patient's total hypermetropia. 

Case 3. Hypermetropic Astigmatism. — Young man 

aged 20. 

R.V.JLHm.4D.=$. 

— _ « 

-T2- 



RET3N0SC0PY 111 

Under atropine, right eye at distant type sees only 
-^y. Fundus reflex very dull, movements of shadow 
slow and against the mirror. On putting up + 5 D. 
the reflex is much brighter, the edge of shadow dis- 
tinct, and its movements quicker. We try -f 6, 7, 8, 
9, and the last gives a shadow moving with the 
mirror. + 8 D. is the highest, which still leaves the 
shadow moving against. On trying the eye at the 
distant type, ■§■ and four letters of -| are at once read. 
No alteration in the glass improves sight. 

Left eye : fundus reflex and movements as in right. 
We commence by trying -f 8 D., which we found the 
other eye required. In the vertical meridian the 
movement is against the mirror, while + 9 D. causes 
it to move with it. In the horizontal meridian with 
+ 8D. the shadow moves with the mirror, and + 7 D. 
causes it to move against. We express it thus : 

+ 8D. 
— + 7 D. ; 

and on trying the combination at the distant type, 

+ 7 D. ep. 

+ 1 D. cylinder axis horizontal, 

the patient is able to read -|; and on decreasing the 
sphere from 7 D. to 6'5 D., -| is read, so that the 
proper correction for this eye is — 

+ 6-5 D. sp. 

+ 1 D. cy. axis horizontal ; 

in this case, therefore, hypermetropia was present in 
one eye, compound hypermetropic astigmatism in the 
other. 



112 TBK REFRACTION OF THE EYE 

4. .mat ism. — Young woman, aged 17, 
sees with either eye ^ — 1 D. = y%. Retinoscopy 
with atropine — 

-3-5D. \ : / -2D. 

r id. L 

\ + lD. 



Ordered guttae atropise sulphatis, gr. iv to 5J> for three 
days ; then with retinoscopy the result i: 



-Em. = —2"5 D. cy. axis horizontal, reads ^. 

L.= y = tID.sp. reads J. 

/\ _ i D . -3D. cy. axis 130' 

After recovering from atropine the result was con- 
firmed, and the following correction ordered to be 
worn constantly : 

p — lD.sp. 

— 25 D. cy. axis horizontal. 

L. - 3 D. cy. :>x\t 130 : . 

Case 5. Mixed Astigmatism. — Mary B — . aged lo. 

pupil-teacher, brought up from Cardiff about her eyee ; 
suffers much from headache and pain in the eyeballs, 
especially the right, worse in the evenings. Has tried 
many opticians to get spectacles to suit her, but has 
always been unable to do so. R. Y. -fy slightly im- 
proved with — ID. L. Y. ^ also slightly impn 
with — ID. On placing the patient in the dark room, 
retinoscopy at on se to be one of m 

_-matism, with the chief meridians horizontal and 



RETINOSCOPY 113 

vertical; we proceed to correct each meridian, and 
the result is — 

-5D. -5D. 



R.— 



— +1D. L.- 



fl-5D. 



On trying this combination before the right eye, T ^- 
is read. We express the vision of right eye thus : 

R.3 6 ¥ + 1D. sp.Q - 6 D. cy. axis horizontal = -^-. 

With the left eye the combination gives, with the 
cylinder not quite horizontal, but slightly outwards 
and downwards, -|. 

L.3% + 1-5 D. sp.Q - 6 D. cy. axis 170° =-|. 

The patient remarked that she had never seen things 
so clearly before. The result was very satisfactory, 
and was arrived at in about ten minutes, thus saving 
an infinite amount of time and trouble, which would 
have been required to work out such a case by any of 
the older methods. Ordered guttse atropiae sulphatis, 
gr. iv to 3J> three times a day for four days, when the 
result was — 

-4D. -4D. 



R.— 



+ 2D. L. 



2D. 



R.V.g% + 2 D. sp.O - 6 D. cy. axis 175° =f. 
L.V. ¥ % + 2 D. sp.O - 6 D. cy. axis 170° = |. 

In this case the glasses were again tried after atro- 
pine was recovered from, and the following glasses 
ordered, which were of course to be worn constantly : 

R +1D. sp. t +1-5D. sp. 

" -6D. cy. axis 175°. — 55 1). cy. axis 170°. 



114 THE REFRACTION OF THE EYE 

Case 6. Mixed Astigmatism. — Mr. C — , aged 24, 
has noticed that for the past few years the eyes 
become very tired at night, especially when much 
writing or reading has been done ; he thinks he sees 
distant objects less clearly than formerly. 

E. Y. -^ not improved with convex or concave 
glasses ; with pin-hole test -f^. 

L. Y. -fg not improved with convex or concave 
glasses ; with pin-hole test -j^-. 

After using atropine for four days, retinoscopy gave 
the following- results : 



R.- 



+ 3D. ,+2-5D. 

— + -5D. L.--+-5D. 



B V + ' 5 D - S P- =$. LV + 5D - 

+ 25 D.cy. axis 160° G ' ' + 1-5 D. axis cy. 165 =f. 

We direct the patient to return after the effects of 
the atropine have passed off, which he does in ten 
days; we then try our correction, deducting + 1 D. 
sphere for the atropine. 

RV. -' 5D - S P- _A L.V. ~' 5D ' _r, 

+ 2-5D. cy.nxisl60 o- 6' + 1-5 D. cy. axis 165° 5* 

This correction was accordingly ordered to be worn 
constantly. 

Case 7. Astigmatism. — Sarah K — , aged 21, com- 
plains that her eyes have of late been very painful, 
and she has also suffered much from headaches, which 
have sometimes ended with an attack of sickness. 

R.v. T <V - 1 D. = T v L.V.£ - 2 D. = T v 



-1-25D. . : y+1J>m 



EETINOSCOPY 115 

After atropine, retinoscopy gave — 

I" 
R.-I- + 1D. 

3-5 D. 

RV +1D - sp - =$■ LV ± 1D iiP: = e 

-225 D.cy. axis horiz. 6 ' -4D. cy. axis 125° 9* 

When the effects of the atropine had passed on 2 , the 
correction which gave the best results was — 

« +-25D. sp. 

R.V. - 2-25 D.cy. axis horiz. =£. L.V. — ns ^-^^=6 

J b -4 D. cy. axisl2o 9' 

These spectacles were ordered to be worn con- 
stantly. 

Case 8. Simple Hypermetropic Astigmatism. — Jane 
Q — , aged 11, has always seen near things badly; she 
turns her head to one side instead of looking directly 
at the object. 

R. V. -£± 110 t improved with spheres, with pin- 
hole -j^-. 

L. Y. 2- 6 T not improved with spheres, with pin- 
hole -j^-. 

Retinoscopy after atropine gives — 



R.- 



+ 1D. +-75D. 
— +5D. L. +4D. 



R.V. +1T) - sp ' =-&-. L.V. + * 75 D - S P- = _6 g 

+ 4 D. cy. axis vert. 12 ' + 3*5 D. cy. axis vert. 1 2 ' 

After the atropine had passed off — 

R.V. +4 D. cy. axis vert. = 3^-. L.V. +3*5 D. cy. axis vert. =■&. 

Spectacles of this strength were ordered for con- 
stant use. 



116 THE EEFEACTION OF THE EYE 

Case 9. Myopic Astigmatism. — Jane P — , aged 23, 
has always seen rather badly, and has had a good 
deal of pain and discomfort in the eves for the past 
six months, especially when nsing them by gas-light. 
at a week ago she noticed, on closing the left eye, 
that the vision of the right was almost gone, though 
she admitted never having tried them separately 
before ; occasionally the right eye turns outwards. 

L.Y.^-1D. = TT . 

H omatropine was applied once, and at the end of 
half an hour retinoscopy gave — 







E.— [ 


- 55 D. 
3D. 




L.— 


I-ID. 

1— ElD. 

! 


With 


glasses 


— 














R.V. 


-3D. up. 


axis 


175'- = 


a 




' - 2-5 D. cy. 


"is* 



L.V.-1D. cy. as:5 5'' = |-. 

This correction was ordered for constant use. 

In most cases thus worked out the glasses may be 
ordered at once, without waiting for the effects of 
the atropine to pass off, — in fact, experience teaches 
that it is a good plan to continue the atropine until 
the spectacles have been made; remembering when 
ordering the correction that in hypermetropia and 
hypermetropic astigmatism the spherical glass will 
require slightly diminishing, usually about 1 D. ; 
in myopia and myopic astigmatism the spherical 
lias to be slightly increased. 



HYPERMETEOPIA 



117 



CHAPTER VI 



HYPEEHETEOPIA 



Hypeemeteopia (H.) (Yntp, in excess ; fifrpov, mea- 
sure ; and w\p, eye) may be defined as a condition in 
which the antero-posterior axis of the eyeball is so 
short, or the refracting power so low, that parallel 
rays are brought to a focus behind the retina (the 
accommodation being at rest). In other words, the 
focal length of the refracting media is greater than 



the length of the eyeball. 



Fig. 66. 




Parallel rays focus at b behiud the retina ; those coming from 
the retina emerge as diverging rays, d, e. 



In the passive hypermetropic eye, therefore, paral- 
lel rays c and a come to a focus behind the eye at b, 
forming on the retina at a a circle of diffusion instead 



118 THE REFRACTION OP THE EYE 

of a point. Bays coming from the retina of such an 

_ having a divergent direction (d and 
these, if prolonged backwards, will meet at k, which 
is the punctuni remotum, and this point being situated 
behind the eye is called negative. 

The distance of the punctum remotum behind the 
eye will equal the focus of the convex lens which 
corrects the hypemietropia ; thus, supposing the p. r. 
situated 20 cm. behind the retina (Vt-- = •-' • 5 D. will 
be the convex glass which will render parallel ray- - 
convergent that they will focus on the retina, or 
cause rays from the retina to be parallel after passing 
through it ; to be mathematically correct, allowance 

Fig. " 




Parallel rays fojussed on the retina by accommodation. The 
dotted line shows the lens more convex as a result of the 
contraction of the ciliary mus : 

must be made for the distance between the cornea 
and the convex lens ; thus, for instance, if the lens 
be placed 20 mm. from the cornea, then the exact 
amount of hypemietropia which the + 5 D. glass will 
correct will be — 

1000 1000 

_ - 20 ~ 180 ~ °° 0, 
In low degrees of hypemietropia the difference is 



HYPEEMETEOPIA 119 

so slight as to be unimportant ; in the higher degrees 
the difference is considerable. 

The hypermetropic eye at rest is only able to 
bring convergent rays to a focus on the retina. All 
rays in nature are divergent, some so slightly so, that 
when coming from a distant object they are assumed 
to be parallel. Rays can be made convergent by 
passing them through a convex lens placed in front 
of the eye ; or the refraction of the dioptric system 
may be increased by the accommodation, so that 
parallel rays may then focus on the retina of a hyper- 
metropic eye. 

Therefore a hypermetrope with relaxed accommo- 
dation see all objects indistinctly. So that such a 
person, having to use some of his accommodation for 
distance, starts with a deficit for all other require- 
ments, equal to the amount of hypermetropia. 

Fia. 08. 




Parallel rays rendered so convergent by passing through a convex 
lens that they focus on the retina. 

Thus, supposing an individual hypermetropic to 
the extent of four dioptres, and possessing 6 D. of 
accommodation, he will, by the exercise of this power 



120 



THE KEFKACTION OF THE EYE 



to the extent of 4 D., be able to bring parallel rays 
to a focus on the retina, and so see distant objects 
clearly; this leaves him 2 D. of accommodation for 
near objects, which "will bring his near point to 50 
cm., a distance at which he will be unable to read 
comfortably. 

Besides, it must be remembered that only a part of 
the accommodation can be used for sustained vision, 
fatigue soon resulting when the whole of the accom- 
modation has to be put in force. 

The following diagram is intended to show the 
amount of accommodation possessed by a hyperme- 
trope of 3D.; each space represents a dioptre, and 
the thick white lines drawn through the spaces give 




the amplitude of accommodation for different ages as 
given on the left of the diagram. The figures above 
indicate the number of dioptres, and those below, the 
near point for each increasing dioptre of accommoda- 
tion. 



HYPERMETROPIA 121 

The amount of hypermetropia is calculated and 
expressed by that convex glass which makes parallel 
rays so convergent that they meet on the rods and 
cones of the retina, the accommodation being sus- 
pended. 

The commonest amount of error is about 2 D. 
Small degrees may require some trouble to discover, 
and can only be found out possibly after the eye has 
been atropized. 

Hypermetropia is divided into latent and manifest. 
The manifest, Donders subdivides into absolute, rela- 
tive, and facultative : 

Absolute, when by the strongest convergence of 
the visual lines accommodation for parallel rays is 
not attained — in other words, when distant vision is 
impaired ; this variety is seldom met with in youug 
people. 

Relative, when it is possible to accommodate for a 
near point, by converging to a point still nearer, — in 
fact, by squinting. 

Facultative, when objects can be clearly seen with 
or without convex glasses. 

In youth the hypermetropia may be facultative, 
becoming in middle age relative, and in old age 
absolute. 

Causes of Hypermetropia : 

1. The antero-posterior diameter of the eyeball 
is too short (axial hypermetropia). This is 
by far the most common cause, and is con- 
genital. 



122 



THE REFRACTION OF THE EYE 



2. A flattened condition of the cornea, the result 

of disease or occurring congenitally. 

3. Absence of the lens (aphakia). 

4. Detachment or protrusion of the retina, owing 

to a tumour or exudation behind it. 

5. A diminution in the index of refraction of the 

aqueous, lens, or vitreous. 



Hypermetropia, therefore, is usually due to shorten- 
ing of the axis of the eyeball. 

The following table shows the amount of shortening 
for each dioptre of hypermetropia, the axial line in 
emmetropia being estimated at 22*824 mm. 

For - 5 of D. of II. there is a diminution in the axial line of "16 mm. 



ID. 
1-5 
2- 
2-5 

3- 
35 
4- 
45 
5- 
6- 
7- 
8- 
9- 
10- 



•31 
•47 
•62 
•77 
•92 

1-06 

1-22 

1-4 

1-6 

19 

2-2 

2-6 

2-9 

3-2 



Hypermetropia is by far the most frequent condi- 
tion of the refraction. It may be looked upon as a 
congenital defect ; frequently also it is hereditary, 
several members of the same family suffering from it. 

Hypermetropia is usually due to an arrest of deve- 



HYPERMETROPIA 123 

lopment, which varies from the slightest degree to 
the extreme condition known as "microphthalmos." 

The following are some of the chief points in which 
the hypermetropic differs from the emmetropic eye : — 
the eye looks small, being less than the normal in all 
its dimensions, especially the antero-posterior ; the 
sclerotic is flat, and makes a strong curve backwards 
in the region of the equator, which can easily be seen 
on extreme convergence, or can be felt by the finger. 
The lens and iris are more forward, the anterior 
chamber is shallow, and the pupil small ; the centre of 
rotation of the eye is relatively further back, while 
the angle a, which is formed between the visual and 
optic axis, is invariably greater, averaging about 7° 
(see p. 202). The result of the large angle a in 
hypermetropia is that the eyes often have an appear- 
ance of divergence, which has sometimes been mis- 
taken for real divergence; whereas in myopia the 
small angle gives to the eyes an appearance of con- 
vergence. 

The ciliary muscle, upon the action of which the 
accommodation depends, is much larger than in em- 
metropia, the anterior portion, which consists chiefly 
of circular fibres, being especially developed; no 
doubt hypertrophied by the constant state of con- 
traction in which it is kept. This contraction is 
called into action by the instinctive desire for clear 
images which all eyes possess, the accommodation 
having to be used for distant as well as for near 
objects. Another result of the constant and exces- 
sive accommodation is that its linked function — the 



124 THE REFRACTION OF THE EYE 

convergence — is liable also to be used in excess; in 
this case an object at a certain distance being accom- 
modated for, one eye will be directed to the object, 
while the other, taking up the excessive convergence, 
will be directed inwards, and so a convergent stra- 
bismus will be produced. To fully understand how 
this convergent strabismus becomes developed, I 
must refer the reader to the chapter on that subject 
(Chap. X). 

When the hypermetropia is of high degree the 
optic nerve is smaller, and contains fewer fibres, so 
that the visual acuteness is frequently below the 
normal. 

Sometimes the face also has a characteristic appear- 
ance, being flat-looking, with depressed nose, the orbits 
being shallow, and the eyes set far apart. Frequently, 
however, there is no distinctive physiognomy. 

The hypermetropic eye is very liable to asymmetry, 
as will be shown when speaking of astigmatism. 

Symptoms of Hypermetropia. — The patient usually 
sees well at a distance, but has difficulty in main- 
taining clear vision for near objects; and since the 
hypermetropia can be more or less corrected by ac- 
commodation, if the error be of a low degree (as 2 or 
3 D.), no ill effects may for some time be noticed; at 
length, however, a point is reached when the accom- 
modation is not equal to long-sustained efforts of 
reading and near work, then accommodative asthe- 
nopia is the result (p. 225). This is especially liable 
to show itself after an illness, or if the patient's 
health has deteriorated from over-work, anxiety, or 



HYPERMETROPIA 125 

other causes. He then complains that after working 
or reading for some time, especially during the 
evenings, the type becomes indistinct, and the letters 
run together ; after resting awhile the work can be 
resumed, to be again shortly laid aside from a 
repetition of the dimness : the eyes ache, feel weak, 
water, etc., frequently headache supervenes ; there is 
a feeling of weight about the eyelids, and a difficulty 
in opening them in the morning. When the hyper- 
metropia is of high degree, the patient may be said 
by his friends to be short-sighted, because when 
reading he holds the book close to his eyes ; by 
doing this he increases the size of his visual angle, 
and thus gets larger retinal images ; this is counter- 
balanced by increase in the circles of diffusion, but as 
the pupils also contract by approaching the book to 
his eyes, some of these are cut off ; so that the advan- 
tage is in favour of holding the book close, especially 
as the patient is probably not accustomed to clear, 
well-defined images. In some cases the ciliary 
muscle contracts in excess of the hypermetropia, so 
that parallel rays focus in front of the retina, and the 
patient therefore presents many of the symptoms of 
myopia : we should always be on our guard against 
such cases. The manner in which the patient reads 
the distant type is often a guide to us in hyperme- 
tropia ; he takes a considerable time to make out each 
line, and yet, if not hurried, eventually reads the 
whole correctly. On looking at the eyes one notices 
that they are red and weak, the lids look irritable, 
and on eversion the conjunctiva is hyperaemic, espe- 



126 THE REFRACTION OF THE EYE 

cially that of the lower lids, while the papilla? are 
frequently enlarged ; the edges of the lids sometimes 
become inflamed and thickened. All these symptoms 
are probably the commencement of troubles which, if 
allowed to go on, may develop into conjunctivitis, de- 
rangements of the lacrymal apparatus, etc., — this 
much we can see ; how much more injurious must be 
the changes which are liable to take place in the in- 
terior of the eyeball from prolonged hyperemia ! It 
cannot be too forcibly insisted on, that in all oph- 
thalmic cases, except those of an acute character, the 
refraction should be taken and recorded as a matter 
of routine, since complaints which prove very intract- 
able are often easily and quickly cured when the 
I roper glasses have been prescribed. 

As the patient advances in age he will become pre- 
maturely presbyopic, so that at thirty-five he may 
suffer from the same discomforts as an emmetrope of 

fifty. 

To test the hypermetropia and measure the amount, 
we take the patient's visual acuteness, each eye sepa- 
rately, and having found that they are alike in their 
refraction, we try the two together ; stronger glasses 
being often borne when both eyes are used, than when 
one is excluded from vision. 

The strongest convex glass with which he is able to 
read ~, or with which he gets the greatest acuteness 
of vision, is the measure of the manifest hyperme- 
tropia (Hm.) . This is not, however, the total hyper- 
metropia, for if the accommodation be paralysed by 
applying a solution of atropiae sulph., gr. iv to %], 



HYPERMETROPIC 127 

three times a day for four days (when we may feel 
sure that not the least vestige of accommodation 
remains) , a much stronger glass can be tolerated, and 
will be required to enable the patient to read -§-. This 
strong glass represents the total hypermetropia, the 
additional amount to that found as Hm. being called 
latent (HI.). 

The following plan is an excellent one for measur- 
ing the manifest hypermetropia. Place in spectacle- 
frames before the eyes such convex lenses as over- 
correct the Hm. ( + 4 D. will usually do this) ; then 
hold in front of these, weak concave glasses, until we 
find the weakest, which thus held in front of +4 D. 
enables -| to be read; the difference between the 
glasses is then the measure of the Hm. By this plan 
the ciliary muscle is encouraged to relax, and we get 
out a larger amount of manifest hypermetropia than 
is obtained by the ordinary method. Thus, supposing 
— 2 D. the weakest glass which, held in front of the 
convex 4 D., enables the patient to read |, + 2 D. 
is the measure of the Hm. (+ 4 D.) + (- 2 D. = 
+ 2D.). 

As age advances the accommodation diminishes, 
and the latent hypermetropia becomes gradually mani- 
fest. Thus a person may have 6 D. of hypermetropia 
latent at ten years of age, 3 of which may have become 
manifest at thirty-five, and the whole of it at about 
sixty-five or seventy, when the total hypermetropia is 
represented by the manifest. 

With the advance of age certain changes take 
place in the structure of the crystalline lens, by 



128 THE REFRACTION OP THE EYE 

which its refraction becomes diminished. This change 
takes place in all eyes, and at a regular rate ; thus at 
fifty-five the refraction has diminished *25 D., at 
sixty-five *75 D., at sixty-eight 1 D., and at eighty as 
much as 2*5 D. Hypermetropia when thus occurring 
in eyes previously emmetropic is styled acquired hyper- 
metropia, in contradistinction to the congenital form, 
which is called original hypermetropia . 

The normal refraction of the eye in early child- 
hood is hypermetropic ; some remain so, a con- 
siderable number become emmetropic as they get 
older, and a certain percentage of these pass on to 
myopia. 

In the diagnosis and estimation of hypermetropia 
several methods are useful. We first estimate the 
acuteness of vision, remembering that being able to 
read | does not exclude hypermetropia, and that we 
must in all cases try convex glasses ; and if the same 
letters can be seen with as without them, then the 
patient certainly has hypermetropia, and the strongest 
convex glass with which he sees them is the measure 
of his Hm. 

We next proceed to retinoscopy ; with this method we 
get a reverse shadow : the quicker the movement and 
the brighter its edge, the lower is the degree of hyper- 
metropia (see p. 95). 

With the ophthalmoscope by the indirect method of 
examination, the image of the disc is larger than the 
emmetropia, and diminishes on withdrawing the 
objective from the eye (p. 69). 

With the mirror alone at a distance, an erect image 



HYPERMETROPIA 129 

of the disc is seen, which moves in the same direction 
as the observer's head (p. 73). 

By the direct method the accommodation of the ob- 
server and observed being relaxed, a convex glass is 
necessary behind the ophthalmoscope, to enable the 
observer to bring the diverging rays from the observed 
eye to a focus on his retina ; the strongest convex glass 
with which it is possible to see the details of the fundus 
clearly, is the measure of the total hypermetropia 
(Kg- 47). 

The treatment of hypermetropia consists, obviously 
in prescribing such convex glasses as will give to rays 
passing through them an amount of convergence, so 
that they will meet on the retina without undue accom- 
modation. It might be thought that, having obtained 
the measure of the total hypermetropia, nothing 
remained but to give such positive glasses as exactly 
neutralise the defect, and that we should then have 
placed the eye in the condition of an emmetropic one. 
Such at first was thought to be the case, though it is 
by no means so, because persons who have been 
accustomed to use their accommodation so constantly, 
both for near and distant objects, as in the case with 
hypermetropes, have very large ciliary muscles which 
they cannot suddenly completely relax ; possibly also 
the elasticity of the lens capsule is somewhat im- 
paired. 

In children and patients under twenty years of age 
it is much better to atropize them at the first, and so 
measure once and for all the* amount of total hyper- 
metropia ; otherwise it will frequently be found that 

9 



130 THE REFRACTION OF THE EYE 

the spectacles have to be constantly changed, the 
asthenopia is unrelieved, and probably the patient has 
to be atropized after all, or becomes dissatisfied and 
goes off to some one else. Another reason in favour 
of atropine is, that with it we cannot possibly mistake 
cases of spasm of the ciliary muscle in hypermetropia 
for myopia, which might otherwise happen, since the 
spasm causes the lens to become so convex that 
parallel rays are even made to focus in front of the 
retina, thus simulating myopia. 

It must always be borne in mind that it is danger- 
ous to atropize patients above the age of thirty-five, 
many well-marked cases of " glaucoma ; ' having been 
traced to the use of this drug; moreover as age 
advances the latent hypermetropia gradually becomes 
manifest, so that the necessity for paralysing the 
accommodation becomes less. 

There exists some difference of opinion among 
ophthalmic surgeons as to the amount of the total 
hypermetropia we ought to correct; some give such 
glasses as neutralise the manifest hypermetropia 
only, while others, after estimating the total, deduct 
perhaps 1 D.from this. It will be found that patients 
vary much as to the amount of correction which is 
most comfortable for them. 

A good practical rule is to prescribe such glasses 
for reading as correct the manifest and one third of 
the latent hypermetropia. 

For example, a child having 6 D. of hyperme- 
tropia of which 2 only are manifest, will require 
+ 3 D. for reading. At the age of twenty, about 



HYPERMETROPIA 131 

4 D. will have become manifest, and the patient will 
then want + 4*5 D. ; at forty, 5 D. will be manifest, 
and he may then be able to bear full correction. 

Hence it will be seen that, as age advances, the 
spectacles will have occasionally to be changed for 
stronger ones, as the latent hypermetropia gradually 
becomes manifest. 

The question arises, should spectacles be worn con- 
stantly or only for near work ? So long as distant 
objects (-§-) can be seen comfortably without them, 
their use is unnecessary except for reading and near 
work; this is generally the case in young persons 
where the hypermetropia does not exceed 3 or 4 D. 
When a convex glass improves distant vision, then 
such can be constantly worn ; somewhat stronger ones 
may be required for reading, etc. ; this is usually the 
case with old people. 

The disadvantage of using spectacles constantly is, 
that after wearing them for some time the patient 
finds he is unable to see without them, which is a 
serious inconvenience ; so that the plan is not to give 
spectacles for constant use until the hypermetropia 
has become relative or absolute. 

In cases of concomitant squint, spectacles which 
correct the hypermetropia are to be worn constantly, 
and here our object must be to give as near the full 
correction as is consistent with the patient's comfort ; 
this we can only find out by experiment in each case. 
The best plan is to measure under atropine the total 
hypermetropia, deduct ID., and give this correction 
for constant use : the reason for making this deduc- 



132 THE REFRACTION OF THE BYE 

tion is that the ciliary muscle is never so completely 
relaxed as when under atropine. 

Convergent strabismus and asthenopia, two of the 
most frequent results of hypermetropia, will be treated 
of in Chapters X and XL 

See Cases 1 and 2, p. 110; also 10, 12, and 17, 
p. 242. 

Aphakia 

Aphakia (' A, priv ; <paKog, lens) is the name given 
to that condition of the eye in which the lens is absent. 
There are several causes, by far the most frequent 
being one of the various cataract operations. Besides 
this aphakia may be caused by dislocation of the lens 
from injury, or dislocation may occur spontaneously, 
and this is probably the cause of those congenital 
cases where no lens can be seen. 

Aphakia necessarily converts the eye into a very 
hypermetropic one. The length of the eyeball which 
would be required (the curvature of the cornea being 
normal and the lens absent) to bring parallel rays to 
a focus on the retina is 30 mm., whereas normally the 
anteroposterior diameter of the eyeballs is only about 
22*8 mm. 

To test aphakia : when a bright flame is held in 
front of and a little to one side of a normal eye, three 
images of the flame are formed, one erect on the 
cornea, another erect on the anterior surface of the 
lens, and a third inverted, and formed on the pos- 
terior surface of the lens. On moving the flame up 
and down, the erect images move with it, and the 



APHAKIA 133 

inverted one in the opposite direction. In aphakia 
two of these images are absent, viz., those formed on 
the two surfaces of the lens. 

Treatment. — Strong convex glasses will be required 
to take the place of the absent lens, the previous re- 
fraction of the eye of course influencing their strength. 
If hypermetropic, stronger glasses will be required ; 
if myopic, weaker. 

The convex glass usually required by an eye pre- 
viously emmetropic, to bring parallel rays to a focus 
on the retina is from 10 to 13 D. 

As every trace of accommodation is lost with the 
lens, stronger glasses will be required for reading or 
near work, and to find out the necessary glass for a 
certain distance, we have only to add to the distance 
glass one whose focal length equals the distance at 
which we wish our patient to see. Thus, if he require 
4 10 D. for distance, and wish to see to read at 25 
cm., we add -f 4 D. to his other glass, and the result- 
ing + 14 D. will adapt the eye to 25 cm. 

The patient may be taught a sort of artificial 
accommodation by moving the spectacles along his 
nose, nearer or farther from the eyes, his working 
point being thereby moved away or brought nearer 
to him. 

In correcting aphakia it will often be found that 
the vision is below the normal. Frequently also there 
is some astigmatism, especially in cases after cataract 
extraction. 

See Case 23, p. 256. 



134 THE EEFRACTEON OF THE EYK 



CHAPTER VII 

Myopia (m.) 

Myopia (Muw ; I close : io\p, the eye), or short -sight, 
is the opposite condition to hypermetropia. 

We saw that the hypermetropic eyeball was too 
short, so that parallel rays f ocussed behind the retina ; 
it is therefore not adapted to any real distance, be- 
cause in order to see any object clearly, it is necessary 
that the defect should be corrected either by the 
accommodation or by means of a convex glass. Now 
in myopia, although the eyeball is too long to allow 
of distant objects being seen clearly, it is perfectly 
adapted for near vision, so that a low degree of 
myopia may not be a very serious disadvantage. 

We spoke of hypermetropia as congenital, due to 
an arrest of development; myopia is an acquired 
defect, and may be looked upon as an effort of nature 
to adapt the eye to near objects, as a result of civili- 
sation and its incessant demands on near vision. 

Myopia is peculiar to the human race, and is met 
with much more frequently in civilised than in un- 
civilised races. 

Low degrees, such as 1 D., may have no very serious 



MYOPIA 135 

drawbacks, because although the full visual acuteness 
can only be obtained by the help of concave glasses, 
many people go half through life, playing cricket, 
tennis, shooting, etc., without finding out the defect ; 
their near vision is really better than that of the 
emmetrope, for they obtain larger retinal images, and 
they have to accommodate less ; against these advan- 
tages it may be stated that many myopes suffer from 
asthenopia, the result of disturbance of the harmony 
between the two functions, accommodation and con- 
vergence, though this disturbance will, of course, be 
more marked in the higher degrees of ametropia. 

Medium degrees of myopia, from 2 to 6 D., are ex- 
ceedingly common; the visual defects are more pro- 
nounced, and it becomes necessary to use glasses for 
many things : often they have to be worn constantly. 
Such patients are liable to suffer from asthenopia, or 
from divergent strabismus and its accompanying evil 
— loss of binocular vision. 

The higher degrees of myopia which increase 
steadily and constantly from an early stage, reaching 
often a very high degree, and carrying in its wake 
destruction and damage to important ocular tissues, 
must be looked upon as a serious disease ; it is desig- 
nated by the name progressive myopia. 

We must now refer to the optical condition of the 
myopic eye. 

Parallel rays, falling on a myopic eye, focus in 
front of the retina, cross and form a circle of diffusion 
(Fig. 70), in place of a clear image. 

Only divergent rays focus on the retina, and hence 



136 



THE EEFEACTION OF THE EYE 



it is necessary that the object looked at be brought 
so near, that rays coming from it are sufficiently 
divergent v^ig. 71), or they must be rendered so by 
asing them through, a concave lens Tig. 12), before 
they fall upon the cornea. 

Via n 




- ». 71. 




Via. :_. 




W may Bay, then, that in myopia the retina 
the conjugate focus of an object, situated at a finite 
distance. The accommodation being at rest, an 



MYOPIA 137 

object situated at this point will be distinctly seen ; 
further off it will be indistinct, nearer it can still be 
seen clearly by putting in force the accommodation. 

The greatest distance at which objects can be seen 
clearly is called the far point (punctum remotum), 
and is always at a definite distance. The higher the 
myopia the nearer to the eye is its punctum remotum 
(p. r.). 

The nearest point of distinct vision is the punctum 
proximum (p. p.), and is determined by the amount 
of the accommodation. To find out the punctum 
proximum, we place in the patient's hand the near 
type, and note the shortest distance for each eye 
separately at which the smallest type can be read, or 
we measure it by the wire optometer in the manner 
before described. The amplitude of accommoda- 
tion is often equal to that in emmetropia, but in the 
higher degrees of myopia it becomes considerably 
diminished. 

The greatest distance at which an object can be 
clearly seen is the exact measure of the myopia ; for 
instance, if the far point be at one metre, a concave 
glass of that strength ( — 1 D.) would render parallel 
rays as divergent as if they came from a distance of 
one metre, and with a glass of this focus the person 
would be able to see distant objects clearly. 

Myopia was for a long time thought to be due to 
an increase in the convexity of the cornea, but as 
a matter of fact the cornea is usually less convex, 
and, as a rule, the greater the myopia the less the 
convexity. 



138 THE KEEKACTION OF THE EYE 

Causes of Myopia : 

1. Too great length of the antero-posterior dia- 

meter of the eyeball (axial myopia). This 
is the common cause of myopia. 

2. Increase of the index of refraction of the lens. 

This may occasionally occnr in the develop- 
ment of cataract. 

3. Conical cornea : this disease simulates myopia 

at its commencement. 

It may therefore be stated that myopia almost 
invariably depends npon a lengthening of the visual 
axis accompanied in many cases by the formation of 
a posterior staphyloma which further increases the 
antero-posterior diameter of the eyeball. This bulg- 
ing, when it occurs, takes place at the outer side of 
the optic nerve towards the macula, and consists 
of an extension backwards with thinning of the 
sclerotic and choroid, and more or less atrophy of the 
latter. 

So constant is this lengthening of the visual axis, 
that from the number of dioptres of myopia can 
be calculated the increase in the length of the eye- 
ball. 

The following table gives the calculation up to 10 D. 



Degree of 
myopia. 


Distance of the p. r. 
in millimetres. 


Inc 
myop 


rea3e in length of the 
c eye in millimetres. 


•5D. 


2000 




•16 


1- 


1000 




•32 


lo 


666-6 




•49 


2- 


500 




•66 


25 


400 




S3 


3- 


333-3 




1- 



MYOPIA 


189 


istance of the p. 
in millimetres. 


r. Increase in length of the 
myopic eye in millimetres 


285-7 


119 


250 


1-37 


222-2 


1*55 


200 


174 


166-6 


2-13 


142-8 


2-52 


125 


293 


111-1 


3-35 


100 


3-80 


ction of a 


myopic eye, in which 



Degree of 
myopia. 

3-5 

4- 
4-5 

5' 
6- 
7- 
8- 
9* 
10- 



the outside measurements were — antero-posterior 
diameter, 30 J mm.; vertical diameter, 25 mm.; trans- 
verse diameter, 25 mm. 

Fig. 73. 




It will be remembered that the emmetropic eye 
measures in the antero-posterior diameter 22'824 mm. 

In Fig. 74 the amount of accommodation is indi- 
cated in a myope of 2 D. by the number of spaces 
through which the thick lines pass; thus at the age 
of thirty the accommodation is equal to 7 D., and the 
near point will be 11 cm.; the distance of the punc- 
tum proximum is given for each dioptre at the 
bottom of the diagram. 



140 THE REFRACTION OF THE EYE 

As the puncturn remotum in myopia is situated at 

a finite distance, therefore, for the same amplitude of 

accommodation, the punctual proximum is nearer the 

eye in myopia than in emmetropia. The near point 

Fig. 74. 

Dioptres. 




Diagram showing the amount of accommodation at different 
ages in a case of myopia of 2 D. 

gradually recedes with advancing age at the same 
rate, whatever the refractive condition of the eye : it 
is clear, then, that the near point in myopia will be 
longer in reaching that point (22 cm.) at which pres- 
byopia is arbitrarily stated to commence than in 
emmetropia, so that in prescribing glasses for pres- 
byopia, the amount of myopia has to be deducted from 
the glass which the emmetrope would require at any 
given age. 

If the myopia amount to 4" 5 D.. then the patient 
can never become presbyopic, because his puncturn 
remotum is only 22 cm. away, bo that lie will always 
be able to see at that distance. Most people imagine 
that those who do not require glasses with advancing 



MYOPIA 141 

age have very strong eyes ; how frequently does one 
hear the remark, when inquiring of a patient's family 
history, " Oh, my father had excellent sight, he 
was able to read at sixty without glasses." This is 
proof positive that he had myopia, though probably 
you will be unable -to -convince the patient of this 
fact. 

In hypermetropia it was shown that the power of 
accommodation had to be used in excess of the con- 
vergence. In myopia we have the opposite defect, 
the patient having* to converge in excess of his accom- 
modation ; thus^, if h£ be myopic 4 D., his far point 
will be at 25 cm. ; ^Klln. looking at an object at this 
distance, it is ne$?fi&ry for him to converge to this 
particular point *hiSangle of convergence being 4, 
while his accommodation remains passive. 

Determining Causes. — The chief factors in the pro- 
duction of myopia are : the constant use of the eyes 
for near work, especially at an early age, when these 
organs are developing ; disturbances of nutrition in 
the tissues of the eye, together in some cases with a 
peculiar conformation of the skull. 

In a large majority of cases myopia is acquired, but 
in a small proportion of cases it may be congenital ; 
this latter form frequently attains a high degree in 
early life, may occur in one or both eyes, and bears 
no relation to the occupation of the patient. Though 
seldom congenital it not infrequently happens that 
one or other of the parents has suffered from myopia. 

There is little doubt that in many cases there is an 
hereditary tendency to it, which, transmitted through 



142 THE REFRACTION OF THE EYE 

several generations, under favourable conditions for 
its development, becomes very decided. 

As in the greater number of cases of myopia the 
factor which tends to produce it is the prolonged use 
of the eyes on near objects, especially while young, 
we may set down myopia as one of the results of 
civilisation and education, and in these days of high 
pressure and competitive examinations it is constantly 
on the increase. The result of the very numerous 
statistics that have been collected, especially by 
German ophthalmologists (myopia in Germany is 
exceedingly common), points to the production of 
myopia in direct proportion to the amount of edu- 
cation. The amount of myopia was found to be 
much greater in town than in country schools, no 
doubt because the general health was better amongst 
those living in the country. Erismann lias come to 
the pleasant conclusion that, if myopia increase in 
the same ratio as it had done during the last fifty 
years, in a few generations the whole population will 
have become " myopic." 

The normal refraction of the eye in childhood is 
hypermetropic ; some tew remain so, a great number 
becoming emmetropic as they get older, and a larger 
percentage of these pass on to myopia. 

In proof of this hereditary tendency to myopia, 
Dr. Colin has summarised the statistics of various 
German writers on this subject. Thus in public 
schools, myopia was found to exist without predis- 
position in 8 per cent., with predisposition in 19 per 
cent. In the higher schools the result was— without 



MYOPIA 143 

predisposition 17 per cent., with predisposition 96 
per cent. 

Residence in towns is also conducive to short- 
sight by causing people to gaze constantly at near 
objects. 

The cause why myopia when once established is 
very liable to increase, is that the extreme converg- 
ence, which is necessary to enable the patient to 
see at the limited distance to which he is confined, 
causes the weakest part of the globe (that part, in 
fact, which is least supported) to bulge, forming a 
posterior staphyloma. In support of this method of 
the production of myopia may be stated the well- 
known fact, that people, such as watchmakers and 
jewellers, who habitually use a strong convex lens 
before one eye, and work at the focal distance of that 
lens, are not especially liable to myopia, proving that 
close work without convergence does not tend to 
produce it. As the eyeball becomes elongated, its 
movements become more difficult, and the pressure 
produced by the muscles during prolonged converg- 
ence tends still further to increase the myopia. 

The stooping position which so many myopes take 
up, causes an accumulation of blood in the eyeball 
which tends to raise the tension as well as materially 
to interfere with its nutrition. Hence results a state 
of congestion, softening, and extension, leading to a 
further increase of the myopia. The more advanced 
these changes, the more difficult is it for the myopia 
to become stationary. 

In addition to these two causes, extreme con- 



144 THE REFRACTION OF THE EYK 

vergence and the stooping position, it is possible that, 
as a result of the constant convergence, the optic 
nerves may be somewhat pulled upon, and thus 
further assist in producing myopia. 

Cases of nebulae, cataract, and other causes of im- 
perfect sight in children may give rise to myopia by 
causing them to hold objects they wish to see close to 
the eyes. 

Symptoms. — The patient sees distant objects badly 
and near objects well. The eyes look prominent; 
the pupils are usually large in young people ; as age 
advances they contract, thus diminishing the circles 
of diffusion, and so slightly improving vision. Ese- 
rine acts in the same manner, so does the nipping 
together of the eyelids, which is so characteristic of 
patients suffering from myopia, and to which the 
disease owes it name. The acuteness of vision is 
frequently below the normal, though objects within 
the patient's far point appear larger than they do to 
the emmetrope, the distance between the nodal point 
and the retina being greater in myopia (Fig. 75). 
This, however, may be partly counterbalanced by the 
stretching of the retina, so that, although the image 
may be somewhat larger, it may not cover a greater 
number of cones than would be the case in an emme- 
tropic eye. 

If the myopia be progressive, frequent limitations 
in the field of vision occur, in the form of scotomata 
due to patches of retinal atrophy. 

Besides seeing distant objects badly, the patient 
complains of pain, fatigue, and intolerance of light, 



MYOPIA 145 

with a state of irritation, especially after using' the 
eyes by artificial light. There may be hyperemia 

Fig. 75. 




a. The retina in an emmetropic eye. B. The retina in a 
myopic eye. C. The visual angle. N. The nodal point. 
The distance from N b is greater than n a, and the image 
of O P is greater at B than at A. 

of the eyes and lids, spasm of the accommodation 
(which increases the apparent amount of myopia), 
pain in the eyeballs on pressure, photopsia, an ap- 
pearance of convergence due to the small size of the 
angle a (p. 202), together with "muscae volitantes." 
These are often a source of great anxiety j the patient 
may, however, be assured that, although they cannot 
be removed, there is no cause for uneasiness; these 
muscae are probably the remains of vitreous cells, 
which, being situated a considerable distance in front 
of the retina, throw shadows on it, and are projected 
outwards as much larger images than would be the 
case in an emmetropic eye ; they appear to the patient 
as black spots. 

The ciliary muscle is smaller than in emmetropia, 
the circular fibres (which are so hypertrophied in 
hypermetropia) being almost absent. 

10 



146 THE REFRACTION OF THE EYE 

The internal recti muscles often act badly, so that 
convergence becomes painful and difficult, often going 
on to divergent strabismus. 

In myopia the convergence has to be used in 
excess of the accommodation ; some patients as they 
become myopic learn to use these two functions in 
unequal degrees, while others are unable thus to dis- 
sociate them ; so that on looking at an object situated 
at the myope's far point, no accommodation and no 
convergence take place, it becomes necessary fchen 
that the two eyes shall make a conjugate movement 
in one direction, so that one eye may receive the 
image of the object on its mucula, while the other eye 
as a result of the conjugate movement has deviated 
outwards, — in other words, divergent strabismus has 
occurred. 

When the myopia is of high degree, the patient 
often uses one eye only for reading, then of course he 
does not require to converge. 

The refraction diminishes slightly with advancing 
age (see p. 128) ; the pupils also become smaller, thus 
cutting off some of the patient's circles of diffusion ; 
so that frequently a marked improvement takes place 
in the vision of myopes as they get older. 

Ophthalmoscopic Appearances. — With the ophthalmo- 
scope, a crescentic-shaped patch of atrophy is fre- 
quently seen on the outer side of the optic disc, 
embracing it by its concave edge ; this is called the 
" myopic crescent." 

In an early stage the crescent looks somewhat 
white, the large choroidal vessels often appear more 






Bale «5r* Duniehson, I.ith. 



MYOPIA 147 

distinct than on the adjoining parts, while gradually 
the blood-vessels disappear, leaving the white scle- 
rotic, which shows up plainly against the red of the 
fundus. Some remains of pigment about the convex 
border of the cresceut are often seen, and frequently 
there is some thinning of the choroid beyond. The 
retina seems to participate in this atrophy much less 
than might have been expected. 

Although the atrophy usually assumes the cres- 
centic form, as shown in Plate 1, which was drawn 
from the fundus of a young man, aged twenty, with 
a myopia of 4 D., yet it may vary much, sometimes 
forming a complete ring round the optic disc (2), or 
it may extend outwards (3), the broadest part being 
always between the disc and the macula. Sometimes 
there is excavation of the atrophic part. 

The optic nerve is occasionally displaced somewhat 
inwards, and the disc, instead of being directed for- 
wards, looks forwards and outwards, making it appear 
oblong in shape from its being seen obliquely (3) ; 
The retinal vessels that pass over the atrophied part 
are often straight in their course, and show up very 
clearly against the white sclerotic. 

The formation of the crescent is much influenced by 
the amount of myopia. In slight degrees in young 
people it is often absent, but in cases of 6 D. or more, 
at the age of twenty, we invariably find a well-marked 
crescent. 

In very high degrees of myopia the epithelial 
layer of the retina atrophies, secondary changes may 
take place in the "yellow spot, as shown in Plate 4 : 



148 THE REFRACTION OP THE EYE 

when such changes take place they cause great 
impairment of vision, due either to extension of the 
atrophy outwards, or to disease commencing there 
independently. If the disease be progressive, the 
vitreous becomes disorganised, with floating opaci- 
ties; the nutrition of the lens may suffer, opacities 
forming in it, especially at the posterior pole ; choroidal 
haemorrhages may occur, and detachment of the retina 
sometimes takes place. 

Further, it may be said that myopes, owing to their 
defective vision, are especially liable to accidents. 

The diagnosis and estimation of myopia is easy. At 
the distant type the patient requires a concave glass 
to enable him to read -J. The weakest lens with which 
he is able to read it is the measure of his myopia ; 
always remember the patient is apt to choose too 
strong a glass if left to himself ; to prevent this and 
enable us to make an exact record of the condition of 
the refraction, by which we may judge if the myopia 
is stationary or progressive, it is much the best plan 
in young people to atropise them in the manner 
previously described. Great differences will be found 
in myopes when testing them at the distant type : in 
some, each increase in the strength of the glass causes 
a corresponding increase of vision; while in others, 
with the same amount of myopia, but little improve- 
ment takes place until nearly the full correction is 
reached, when it suddenly becomes almost normal : 
hence it is not sufficient after trying two or three 
concave glasses without any visual improvement, to 
at once assume the absence of myopia. On placing 



MYOPIA 149 

the near type in his hand, he will be found to be 
able to read the smallest print, though at a shorter 
distance than that for which it is marked. The 
extreme distance at which he is thus able to read it 
is his far point, the measure of which is also a measure 
of his myopia ; this is a most useful guide to us : for 
instance, he reads No. 1 at 20 cm. but no farther; 
1 2^-° = 5 D., therefore 5 D. is the measure of the 
myopia, and such a glass will render parallel rays so 
divergent that they will seem to come from 20 cm. 
Had he been able to read it at 10 cm. only, then 
(i_oo _ jo D.) - 10 D. would be the measure of the 
myopia. 

With retiuoscopy the shadows move in the same 
direction as the concave mirror so long as the observer 
is beyond the patient's far point (p. 90). 

With the ophthalmoscope, by the indirect examina- 
tion, the disc looks smaller than in emmetropia, and 
becomes larger on withdrawing the objective farther 
from the eye (p. 70). 

With the mirror alone at a distance, an inverted 
magnified image of the disc can be clearly seen, 
provided always that the observer be not nearer 
the aerial image than his own near point (Fig. 45). 
The lower the myopia the greater the image, because 
the longer is the distance between the image and 
tho myopic eye. On moving the head from side to 
side the image will always move in the opposite 
direction, showing that it is an inverted one. 

By the direct method of examination the fundus can- 
not be clearly seen until a concave glass is placed in 



150 THE REFRACTION OF THE EYE 

front of the observing eye. The weakest concave glass 
with which the details of the macula and disc can be 
clearly seen (the observer's eye being emmetropic 
and the accommodation relaxed) is a measure of the 
myopia (Fig. 48). This test may be relied upon for 
the lower, but not for the higher degrees of myopia. 

The treatment of myopia. — The chief indications 
are — 

1st. To prevent the increase of the myopia. 

2nd. To enable the patient to see well. 

3rd. To prevent the various troubles from which 
myopes are so liable to suffer, as asthenopia, divergent 
strabismus, etc. 

To carry out the first of these indications, strong 
convergence and the stooping position, which play 
so important a part in the production of myopia, 
must be avoided, the patient being directed never to 
read in a train or carriage, where every movement 
requires a change in the accommodation, nor to look 
at near objects for too long together : the natural 
tendency for a myope who is excluded in great mea- 
sure from seeing distant objects, is to devote himself 
to near ones. In reading, writing, or working, he 
must keep 35 cm. away from the book or paper, use 
books printed in good bold type, and not write too 
small, while the desk and seat should be conveniently 
arranged so as to avoid stooping. He should do as 
little as possible by artificial light; when necessary, 
it is best to use a reading lamp, so placed that it 
throws the light down upon the work, leaving the 
remainder of the room in comparative darkness, so 



MYOPIA 151 

that when the eyes become tired they may be rested 
by turning them from the light. The stooping 
position must be strictly avoided, as it causes an 
increased flow of blood to the interior of the eyeball, 
and at the same time, by compressing the veins in 
the neck, obstructs the returning blood, and so pro- 
duces hypersemia with symptoms of irritation, and 
possibly some slight increase of tension. When 
reading or writing he should sit with his back to the 
window, so that the light may fall on the book or 
paper over his left shoulder, the shadow of his pen 
being thus thrown to the right, enabling him to see 
plainly the letters he is forming. 

Attention must be paid to the general health; 
iron internally often being especially useful, combined 
with regular outdoor exercise and good nutritious 
food. 

When symptoms of irritation show themselves, 
with a rapid increase in the myopia, complete rest 
must be given to the eyes, and in no way can this be 
so conveniently carried out as by dropping into the 
eyes a solution of atropine (gr. j to Jj) three times a 
days, for some two or three weeks ; counter-irritation 
may be applied to the temples and behind the ears in 
the shape of small blisters, or by a solution of 
iodine : no spectacles must be allowed. Sometimes 
where there are symptoms of congestion present, the 
artificial leech applied to the temple once a week 
for a few weeks does much good. As the irritation 
gradually subsides, the patient may be allowed to 
do a little reading daily in a good light, the eyes all 



152 THE REFRACTION OF THE EYE 

the time being kept under atropine ; he may require 
glasses to enable him to do this. Thus if he have 
myopia of 3 D. he will not require them, his far point 
being at 33 cm.; if he has — 1*5 D. he will require + 
1*5 D. to enable him to read at about 33 cm. ( + 3 D.) 
+ ( - 1-5 D.) = + 1-5 D. ; if the myopia is 6 D. he 
will require — 3 D. to put back his far point from 16 
to 33 cm. ( + 3 D.) + ( - 6 D.) = - 3 D. 

So long as the myopia is progressive it must always 
be a source of anxiety to us. 

To enable the patient to see well both near and 
distant objects, as well as to prevent extreme conver- 
gence, we must correct the myopia. In young people 
with good accommodation and with a low degree of 
myopia the full correction may be well borne, the 
patient wearing such glasses constantly; and it has 
been observed that in those who from their youth 
have worn their full correction constantly, for both 
near and distant objects, the myopia has usually 
remained stationary. 

There are two exceptions to this general rule of the 
full correction of myopes : 

1st. Where the myopia is of high degree, and the 
acuteness of vision is reduced, then the concave 
glasses so much diminish the size of the retinal 
images, that the individual is induced to make these 
images larger by bringing the object closer. 

2nd. When the myopia is of high degree, and the 
patient has, from long custom, become used to 
exercise the function of convergence in excess of his 
accommodation, the full correction, which gives him 



MYOPIA 153 

perhaps excellent distant vision, causes him pain 
when used for near objects. Here we must give two 
pairs of spectacles, one for distant vision, and the 
other for near objects ; the latter may be gradually- 
increased in strength as the patient becomes accus- 
tomed to them, so that after a time, possibly a year 
or so, the full correction may be comfortable for 
constant use. 

In those cases where the myopia is of high degree, 
and the patient is unable to bear the full correction 
for reading", we find out the necessary glass by sub- 
tracting from the lens which gives the best acuteness 
of vision, that glass whose focus represents the dis- 
tance at which the patient wishes to read or work. 
Thus, for example, — 9 D. gives the best distant 
vision ; the patient wishes for glasses with which to 
read at 33 cm. (- 9 D.) -f- (+ 3 D.) = - 6D.); - 6 D. 
will be the glass required, and will enable the patient 
to read at 33 cm. without using his accommoda- 
tion. 

Glasses may also be required for music. When the 
myopia is of low degree, and we are certain that the 
disease is stationary, folders may be allowed for dis- 
tance, no glass being used for near work. 

Single glasses are occasionally allowed in low 
degrees of myopia for looking at distant objects ; 
they have the disadvantage that they encourage the 
patient to give up binocular vision, and may so assist 
in the development of a divergent squint. 

When muscular asthenopia is present, prisms with 
their bases inwards (which diminish the necessity for 



154 THE REFEACTION OF THE EYE 

convergence), with or without concave glasses, are of 
great value. 

When photophobia is a prominent symptom tinted 
spectacles may be comfortable (p. 235). 

It is important to impress on the patient that the 
glasses for reading are not given to enable him to 
see better, but to increase the distance at which near 
work can be done. 

When the myopia has been estimated under atro- 
pine, it is often necessary to add on to the glass so 
found — "5 D., as the full correction under the 
mydriatic is usually this much weaker than the cor- 
rection found without it, the reason being that the 
ciliary muscle is never so completely relaxed as it is 
by atropine. 

I am of course aware that the above optical treat- 
ment of myopia is at variance with the teaching of 
French authorities. 

Landolt considers that the action of the ciliary 
muscle may have a tendency to increase the myopia, 
and therefore states that myopes should never wear 
glasses which require the patient to use his accommo- 
dation : so that in low degrees of myopia glasses are 
only allowed for distant objects ; in medium degrees, 
glasses which under-correct the myopia are given for 
near objects, so as to enable the wearer to see at a 
given distance without accommodation. 

My own opinion is, that every case requires treating 
on its own merits ; very many myopes wear their full 
correction constantly with comfort, and if not with 
benefit to the eyes most certainly without injury ; 



. MYOPIA 155 

while other myopes will occasionally be found who 
suffer from asthenopia when using their full correction 
for near vision. In extreme degrees of myopia, and 
in those where the disease is increasing rapidly, rest 
for the eyes, and not spectacles, is the essential 
treatment. 

In cases of high myopia (over 15 D.) the lens may 
be removed by a needle operation followed by curet- 
ting, and thus the eye may be brought nearly to the 
point of emmetropia, the patient getting good distant 
vision without glasses. Many of these cases have 
given most gratifying results. Usually only one eye 
is operated upon. 

See Cases 11 and 19, pp. 245 and 253. 



156 THE REFRACTION OF THE EYE 



CHAPTER VIII 

ASTIGMATISM AND ANISOMETROPIA 

Astigmatism ('A, priv ; orfyjua, a point) . 

Hitherto we have seen that the cornea usually 
takes but little part in the defects we have been con- 
sidering. It has been shown that hypermetropia is 
almost invariably due to the eyeball being too shorts 
and myopia to its being too long. We now come to 
a defect in which the curvature of the cornea plays a 
very important part, with or without some decrease 
or increase (from the emmetropic standard) in the 
antero-posterior diameter of the eyeball; I refer, of 
course, to astigmatism, which is the commonest of 
all the refractive errors, few cases of hypermetropia 
being entirely free from it, and still fewer cases of 
myopia. Astigmatism may be denned as that state 
in which the refraction of the several meridians of 
the same eye is different : for instance, the vertical 
meridian may be emmetropic, the horizontal hyper- 
metropic. 

Astigmatism is usually congenital, but may be 
acquired ; frequently there is some hereditary ten- 
dency. 



ASTIGMATISM 157 

Astigmatism was first discovered by Thomas Young 
in 1793, who was himself astigmatic. 

Astigmatism may be divided into two chief varie- 
ties : 

1. Irregular. 

2. Regular. . 

Irregular astigmatism consists in a difference of 
refraction in the different parts of the same meridian, 
and may be further subdivided into normal and 
abnormal, (a) Normal irregular astigmatism is due 
in great measure to irregularities in the refracting 
power of the different sectors of the lens ; it causes a 
luminous point to appear stellate, as in the case of a 
star, which is, in reality, round, (b) The abnormal 
variety may arise from the condition of the lens or 
of the cornea : when the lens is at fault, it may be a 
congenital defect, or it may be acquired from changes 
taking place in the lens itself ; or it may result from 
partial displacement. The changes in the cornea 
which may produce it are, conical cornea, nebulas, and 
ulcers. Little can be done in the way of glasses 
towards correcting this form of astigmatism, though 
much improvement of vision sometimes occurs when 
stenopaic spectacles are worn, the opening being 
made to suit the peculiarity of each case. 

We now pass on to the much more common variety, 
which can frequently be exactly corrected by the help 
of piano-cylindrical lenses. 

Regular astigmatism is due to the curvature of the 
cornea being different in the two meridians, that of 
maximum and minimum refraction j these are called 



158 THE REFRACTION OF 1HE EYE 

the chief meridians, and are always at right angles to 
each other. 

In the normal eye the cornea is the segment of an 
ellipsoid and not of a sphere, so that there is a slight 
difference in the refraction of the two chief meridians, 
the focus of the vertical meridian being slightly 
shorter than that of the horizontal. 

This can easily be proved by looking at a card on 
which is drawn two lines crossing each other at right 
angles ; the card is held close to the eye and gradually 
made to recede; both lines cannot be seen at the 
same time with equal clearness, the horizontal being 
seen clearly at a shorter distance than the vertical 
line. So long, however, as the acuteness of vision is 
not impaired it goes by the name of normal astig- 
matism, or regular astigmatism of the normal eye. 

Parallel rays passing through a convex spherical 
glass come to a focus at a point. If the cone of light 
thus formed be divided perpendicular to its axis, at 
any point between the lens and its focus, or beyond 
the focus after the rays have crossed and are diverg- 
ing", a circle is formed. In astigmatism the case is 
different : if parallel rays pass through a convex lens 
which is more curved in the vertical than in the 
horizontal meridian, those rays which pass through 
the vertical meridian come to a focus sooner than 
those which pass through the horizontal; and the 
resulting cone, instead of being circular as in the pre- 
vious ease, will be more or less of an oval, forming a 
circle only at one point (4, Figs. 16 and 77). Let as 
now divide this cone at different points at right angles 



ASTIGMATISM 



159 

diffusion 



to its axis, and notice the shape of the 
patches thus produced. 

At 1, an oblate oval is formed; at 2, a horizontal 
straight line, the rays passing through the vertical 
meridian having come to a focus; at 3, 4, 5, the rays 



Fig. 76. 




Fig. 77. 




Section of cone of light at 1, 2, 3, 4, 5, 6, 7, Fig. 76. 

passing through the vertical meridian have crossed 
and are diverging, and the rays passing through the 
horizontal meridian are approaching; at 4 a circle is 
formed ; at 6 a vertical straight line, the rays passing 
through the horizontal meridian have met, while those 
passing through the vertical meridian are still diverg- 
ing; a large prolate ellipse is formed at 7. 

The space between h and v, h being the point at 



160 



THE REFRACTION OF THE EY 



which the rays passing through the horizontal meri- 
dian focus, and v the point at which the rays passing 
through the vertical meridian meet, is called the 
interval of Sturm (i, Fig. 78). 

Regular astigmatism was at one time thought to 
be due to defects in the curvature of the lens, but it 
has since been proved to depend almost entirely on 

Fig. 78. 




asymmetry of the cornea. The lens may, however, 
influence it in two ways : — 1st. Its two chief meri- 
dians may not correspond to those of the cornea. 
2nd. Owing to the position of the eye the lens may 
be situated obliquely. 

It has been experimentally proved that slight 
amounts of corneal astigmatism may be corrected or 
disguised by the unequal contraction of the ciliary 
muscle (one segment of the muscle acting while the 
rest of the circle remains passive); the curvature of 
the lens is thus increased in the direction of the 
ciliary contraction only. 

In astigmatism the vertical meridian of the cornea 
has usually the maximum, and the horizontal meri- 
dian the minimum of curvature, corresponding to the 
astigmatism of the normal eye, when this is so, we 



ASTIGMATISM 



161 



speak of it as astigmatism according to the rule. To 
this, however, there are numerous exceptions. Thus 
the chief meridians may occupy an intermediate 
position, or the vertical may have the minimum, and 
the horizontal the maximum of curvature, then we 
have astigmatism against the rule. Whatever the 
direction of the two chief meridians, they are always 
at right angles to each other. 

There are five varieties of regular astigmatism : 

1. Simple hypermetropic astigmatism. 

2. Compound hypermetropic astigmatism. 

3. Simple myopic astigmatism. 

4. Compound myopic astigmatism. 

5. Mixed astigmatism. 

In the first variety, one set of rays (we will assume 
the vertical, v) have come to a focus on the retina, 
while those at right angles, the horizontal (h), focus 
behind the eye. Thus, instead of a point, as in em- 
metropia, a horizontal straight line is formed on the 
retina, Fig. 79. 

Fig. 79. 




In the second variety, both sets of rays focus be- 
hind the retina, forming an oblate oval (Fig. 80) . 

In the third variety, one set of rays (we will assume 
the vertical) focus in front of the retina, the other set 

11 



162 



THE REFRACTION OF THE EYE 



on the retina, thus forming a vertical straight line 
instead of a point (Fig. 81). 

In the fourth variety, both sets of rays focus in 



Fig. 80. 




Fig. 81. 




front of the retina, forming an upright oval (Fig. 
82). 



Fig. 82. 




In the fifth variety, one set of rays has its focus in 
front, and the other set behind the retina (Fig. 83). 



ASTIGMATISM 163 

In these five figures, the focus of the vertical 
rays has been placed in front of the focus of the 

Fig. 83. 




horizontal rays ; of course, it will be understood 
that the position of these two foci are frequently re- 
versed. 

From what has been said it will easily be seen, that 
when an astigmatic eye looks at a spot, it sees not a 
spot, but a line, an oval, or a circle ; hence its name 
(a and aTiyfia). 

It is necessary that it should be thoroughly under- 
stood how the image of a line is formed on the retina : 
the clear perception of a line depends upon the dis- 
tinctness of its edge, and to gain a clear image of this 
line it is necessary that the rays coming from a suc- 
cession of points which make up this line (they of 
course emerge in every direction) should be brought 
to a focus on the retina, having passed through the 
cornea at right angles to the axis of the line. Should 
they not do so, circles of diffusion are formed, which 
overlap each other and so render the edges ill-defined. 
The rays which diverge from the line parallel with its 
axis, overlap each other on the retinal image, in- 



164 THE REFRACTION OF THE EYE 

creasing its clearness, except at the extremities, 
where they overlap and cause some slight indistinct- 
ness. Thus a person with simple astigmatism, myopic 
in the vertical and emmetropic in the horizontal, sees 
distinctly vertical lines, because the rays coming from 
the edges of the vertical line pass through the hori- 
zontal or emmetropic meridian, while those which 
come from the line parallel with its axis pass through 
the myopic meridian and overlap each other without 
causing any indistinctness of its edges. Therefore 
a patient with simple astigmatism sees clearly the line 
which is parallel with his ametropic meridian, and 
indistinctly the line parallel with his emmetropic 
meridian. 
Causes : 

1. Congenital malformation of the cornea, which 

may in astigmatism of high degree be part 
of a general malformation of the face and 
skull. 

This variety of astigmatism usually re- 
mains unchanged throughout life. 

2. Operations involving the cornea or sclerotic, 

such as cataract extractions, iridectomy, etc. ; 
these operations often cause by their cicatri- 
sation a high degree of astigmatism, which 
changes considerably with time. 
Symptoms. — There is frequently a want of sym- 
metry about the patient's head or face. If young, 
and the astigmatism hypermetropic and of low degree, 
few symptoms may be present ; usually, however, the 
patient complains of defective vision, with asthenopia, 



ASTIGMATISM 165 

especially if his work be such that his accommoda- 
tion is in constant use ; sometimes headache is a very- 
marked symptom, either frontal or occipital ; he has 
probably tried all sorts of spectacles, and can find none 
to suit him. On trying him at the distant type, his 
acuteness of vision is always below the normal, the 
mixed variety of astigmatism affecting it most, and 
next the compound. The way the patient reads the 
type may be an indication of the defect : he may be 
able to read certain letters better than others ; thus 
he may decipher some letters of -^ only, and yet 
be able to read some of -£% and even some of -|. 
We sometimes notice, when trying the acuteness of 
vision, that the patient sees much better if allowed 
to hold his head on one side ; by doing this he places 
his nose somewhat in the line of vision of the eye 
he is using, and so cuts off some of the rays which 
would otherwise enter his pupils, thus diminishing 
his circles of diffusion. It is possible that if his chief 
meridians are oblique, by thus tilting them he brings 
them to correspond with the meridians of the object 
looked at. Whether this explanation be the correct 
one I know not, but we may generally feel pretty 
confident, when we see the patient looking at the 
test-type with his head on one side, that astigmatism 
is present. One frequently hears it said that images 
formed on the retina in astigmatism are distorted; 
this, however, is not the case, as can readily be proved 
by making one's own eye astigmatic, by placing in 
front of it a cylindrical glass : a certain amount of 
blurring and indistinctness is produced, but no actual 



166 THE REFKACTION OF THE EYE 

distortion, the distance between the cornea and retina 
being insufficient. 

Usually both eyes are affected, sometimes quite 
symmetrically. Frequently, however, there is a great 
difference, one eye being almost emmetropic, the other 
very astigmatic. 

In astigmatism, when the chief meridians of one 
eye are at right angles to the chief meridians of the 
other, binocular may be much better than monocular 
vision : we will illustrate this by a simple example. 
The right eye we will assume to be hypermetropic 
2 D. in the vertical meridian, emmetropic in the hori- 
zontal ; the left emmetropic in the vertical, hyperme- 
tropic in the horizontal 2 D. We know that the 
patient, looking at the fan of radiating lines with the 
right eye only, will see the vertical lines distinctly, 
the horizontal only by accommodating ; with the left 
eye the horizontal lines will be clearly seen, the ver- 
tical ones indistinctly; with the two eyes all the 
lines will appear fairly distinct, the image in one eye 
overlapping that of the other. We seldom find a 
case in which the correction is so complete as in our 
example, but we meet with cases where partial cor- 
rection takes place. 

In my experience vision is less impaired when the 
chief meridians are vertical and horizoutal than when 
they are oblique. 

As hypermetropia is more common than myopia, 
so also is hypermetropic astigmatism of more frequent 
occurrence than the myopic variety, though few 
myopes will be found who are quite free from astig- 



168 TEST TYPES 



PRAT'S TEST TYPES FOR ASTIGMATISM. 



Horizontal. 15° 30° 



45° G0° 






« m # v \v 



90° 105° 120° 

lllnll %J O 



135° 150° 165 { 



ASTIGMATISM 169 

matism. Mixed astigmatism is the least frequently- 
met with. 

If, after trying the patient at the distant type, we 
are not satisfied with the result, though perhaps we 
have some improvement with either convex or con- 
cave glasses, we may suspect astigmatism and pass 
on to some of the special tests by which it may be 
diagnosed and estimated. 

If astigmatism exist, our first object must be to 
find out the direction of the two principal meridians, 
viz. those of maximum and minimum refraction. 

Most of the tests for astigmatism are based upon 
the principles of the perception of a line. An astig- 
matic eye looking at a test object composed of lines 
radiating from a centre, and numbered for conve- 
nience like the face of a clock, is unable to see all the 
lines equally clearly. The line seen most distinctly 
indicates the direction of one of the two chief 
meridians; the other chief meridian being of course 
at right angles to the one most clearly seen. The 
fan of radiating lines now very commonly used, as 
well as the clock face with movable hand, are 
all convenient test objects. The striped letters of 
Dr. Pray are useful for indicating one of the chief 
meridians. 

To test and measure the astigmatism, we place our 
patient at a distance of six metres in front of the 
clock, Fig. 84, covering up one eye with a ground- 
glass disc. Supposing he see plainly the three lines 
from 12 to 6, all the other lines being more or less 
indistinct, those from 3 to 9 most so ; and further, if 



170 THE REFRACTION OP THE EYE 

on placing before the eye a weak positive glass we 
find that lines from 12 to 6 are blurred, we know then 

Fig. 84. 




that the horizontal meridian — that is, the meridian 
at right angles to the clearly defined line — is emme- 
tropic, as well as being one of the principal meridians. 
We now direct him to look steadily at the lines from 3 
to 9, i. e. those at right angles to the lines first seen; 
we try what spherical glass enables him to see these 
lines distinctly and clearly ; this glass is the measure 
of the refraction of the vertical meridian, and there- 
fore also of the astigmatism. 

To obtain reliable results, the eye must be tho- 
roughly under the influence of atropine. 

Supposing lines from 12 to G be clearly seen, but 



ASTIGMATISM 171 

that with a weak convex glass they are blurred ; and 
that on looking at lines 3 to 9 no convex glass im- 

Fio. 85. 




proves their clearness, while —ID. renders them 
quite distinct, the case is one of simple myopic astig- 
matism. 

With the ophthalmoscope the astigmatism may also 
be recognised. 1st. With the indirect method we 
find that the shape of the disc, instead of being cir- 
cular, is more or less oval, changing its shape as the 
objective, which must be held exactly perpendicular, 
is withdrawn. 2nd. With the direct method we find 
that the disc appears oval, the long axis of the oval 
corresponding to the meridian of greatest refraction. 
Figs. 86 and 87 show the same disc as seen by the 
direct and indirect examination. 

It is, however, the difference in degree of the clear- 
ness of the retinal vessels that is to be taken as the 
guide, not only of the chief meridians, but also of the 
kind and amount of error. To detect this, assuming 



172 



THE REFEACTION OF THE EYE 



that the chief meridians are vertical and horizontal, 
we take notice first of the lateral margins of the disc, 



Fig. 86.* 



Fig. 87. 





Erect image. 



Inverted image. 



and of a vessel running in the vertical direction, and 
find out the strongest positive, or the weakest negative 
glass, with which these are distinctly seen, using a re- 
fracting ophthalmoscope. We then take a horizontal 
vessel with the upper and lower margins of the disc, 
and estimate their refraction in the same manner. 
Thus a vessel going upwards is first taken ; it is seen 
well with convex 1, the horizontal meridian therefore 
is hypermetropic J D. A horizontal vessel is now 
looked at, and can be best seen with concave 1, show- 
ing that the vertical meridian is myopic one dioptre ; 



* I have to thank Mr. Nettleship for these woodcuts from his work 
on ■ Diseases of the Eye.' 



ASTIGMATISM 1 73 

the case is, therefore, one of mixed astigmatism. 
"When the chief meridians are not vertical and hori- 
zontal, we must endeavour to find a vessel which 
coincides with one of the chief meridians, and having* 
estimated this, we look for a vessel at right angles 
to that first chosen, and find out its refraction in the 
same way; this gives us the other chief meridian. 

3rd. Retinoscopy. This is, I think, the easiest and 
most trustworthy of all the objective methods. The 
patient being fully atropised, the principal axes can 
be seen at a glance, and the proper glasses for cor- 
recting the error easily found by anyone who has 
taken the trouble to familiarise himself with this 
method of examination. For a full description of 
retinoscopy the reader must refer to Chap. Y, p. 82. 

Astigmatism requires for its correction a cylindrical 
glass, and reference has already been made to such a 
lens on p. 32. 

This cylindrical glass is the segment of a cylinder ; 
whereas a spherical glass is the segment of a sphere. 
The cylinder may be either concave or convex, and is 
numbered according to the refraction of the meridian 
of greatest curvature ; the result upon rays that pass 
through it is, that those which pass through parallel 
to its axis undergo no refraction ; all other rays are 
refracted, those most so which pass at right angles 
to the cylinder. A cylinder thus possesses the power 
of exactly neutralising the astigmatism. 

On referring back to Fig. 79, which represents a 
case of simple hypermetropic astigmatism, the vertical 
meridian being emmetropic and the horizontal meri- 



174 THE REFRACTION OF THE EYE 

dian hypermetropic, it will be seen that a convex 
cylinder can be found, which with its axis vertical 
will increase the refraction of rays passing through 
the horizontal meridian, so that they meet exactly on 
the retina. Suppose the glass required be + 1 D. 
cylinder, this not only corrects, but is itself a measure 
of the astigmatism. If a patient with astigmatism of 
1 D. be able to read T ^ of the distant type and with 
the cylinder + 1 D. axis vertical -[j, it may be ex- 
pressed in the following manner: T r v + 1 D. cy. axis 
vert. = f 

Fig. 80 represents compound hypermetropic astig- 
matism. We find out the refraction of each chief 
meridian by retinoscopy or the clock face. Assuming, 
then, the vertical meridian to be + 1 D., and the 
horizontal + 2 D., if we place our positive cylinder + 
1 D. with its axis vertical, we shall have corrected 
the astigmatism, and the error will be reduced to one 
of simple hypermetropia, requiring for its correction 
+ 1 D. sphere. This combination of sphere + 1 D. 
with cylinder + 1 D. axis vertical is made in one 
glass, by the optician grinding upon one side the 
sphere + 1 D. and on the other the cylinder + 1 D. 
The lens thus formed is called a spherico-cylindrical 
lens. 

Fig. 81 represents simple myopic astigmatism, in 
which the vertical meridian is myopic and the hori- 
zontal emmetropic. To correct this error it is neces- 
sary to cause the rays which pass through the vertical 
meridian to be so refracted that they meet at instead 
oi, in front of the retina. Here it is obvious that a 



ASTIGMATISM 175 

negative cylinder with its axis horizontal will accom- 
plish this object. 

Fig. 82 represents compound myopic astigmatism. 
Both sets of rays focus in front of the retina, one 
set in advance of the other. This is corrected by 
carrying the focus back by a negative sphere, and so 
reducing the case to one of simple myopic astig- 
matism, which is corrected by a negative cylinder. 
This glass is called a negative spherico-cylindrical 
lens. 

Fig. 83 represents mixed astigmatism. One set of 
rays focus in front of the retina, the other set behind 
it. The difference between these is the amount of 
astigmatism, and may be corrected in three different 
ways. Thus supposing the vertical meridian myopic 
1 D., and the horizontal hypermetropic 1 D., the 
correction may be made by — 1 D. cylinder axis 
horizontal, which puts back the vertical rays so as to 
focus on the retina, combined with a + 1 D. cylinder 
axis vertical, which brings forward the horizontal 
rays to the retina. This compound lens is called a 
concavo-convex cylinder. There are, however, some 
difficulties in using this method of correction; the 
axes of the cylinders have to be arranged with such 
exactness, that the slightest variation may upset the 
whole result. Besides, it is difficult, when using such 
a combination at the distant type, to make alterations 
with the same facility with which one does other 
combinations. Moreover, during the grinding, very 
great care is required of the optician; so that 
either of the following plans seems preferable : by 



176 THE REFRACTION OF THE EYE 

a concave spherical glass of 1 D., combined with 
a convex cylinder of 2 D. axis vertical ; or by a 
+ 1 D. sphere, combined with — 2 D. cylinder axis 
horizontal. 

Treatment. — Having found out by one of these 
numerous methods the refraction of the two chief 
meridians, we confirm the result by trying the patient 
at the distant type with the combination so found, 
making any slight alterations which may be necessary. 
These glasses may be ordered at once, remembering, 
when atropine has been used, that in hypermetropic 
astigmatism we must reduce the convex sphere about 
1 D., while in the myopic variety the concave sphere 
must be slightly increased by about "5 D. 

TTe frequently have to be satisfied with glasses 
which do not raise the vision to -|, and if such have 
been carefully chosen, we often find that after they 
have been worn for some time the vision improves, 
due no doubt to the retina becoming: more sensitive to 
well-defined images, a condition of things to which it 
was previously unaccustomed. 

In ordering- glasses for astigmatism, we must be 
careful to give the exact axis of each cylinder : opti- 
cians supply us with convenient forms, having the 
diagram of a frame marked in degrees : we indicate 
the axis by drawing a line through this diagram. 

The Ophthalmometer of Javal and Schiotz is an 
instrument for measuring the amount of corneal 
astigmatism. Scientifically it may be of much value, 
as by it we are enabled to separate astigmatism due 
to the cornea from that due to the lens; but the 



ASTIGMATISM 177 

price will prevent its coming into general use, espe- 
cially as we possess so many other methods by which 
astigmatism may be estimated, and probably the 
separation of the two forms of astigmatism is a dis- 
advantage practically, when we are seeking to correct 
the defect. 

With the ophthalmometer two objects are reflected 
on to the cornea of the observed eye; these objects 
are of white enamel, one quadrilateral in shape, the 
other of the same size except that on one side it is 
cut out into five steps : these two objects slide on 
a semicircular arm, which rotates round the tube 
through which the observer looks, one object on 
either side of the tube ; the observer looking through 
this tube, which contains a combination of convex 
glasses and a bi-refracting prism, sees four magnified 
images in a line on the cornea under examination. 
First find out the meridian of least refraction; this 
we are able to do by finding the position of the semi- 
circular arm, in which the two central images (one 
quadrilateral, the other with steps) are furthest apart. 
We slide the two objects together until we see the 
two central images on the observed cornea just 
touch, the lowest step of the one with the side of the 
other ; this, then, is the meridian of least refraction, 
and we note it down as such. Now turn the arm at 
right angles to this meridian, and notice the amount 
of overlapping of the two central images ; each step 
in the one figure that is overlapped by the quadri- 
lateral one is equal to one dioptre. Thus if it over- 
lap three steps, there is a difference of 3 D. between 

12 



178 THE REFRACTION OF THE EYE 

the meridians of least and greatest refraction; we 
know this to be the meridian of greatest refraction, 
because it is at right angles to the one first found. 

As there are only five steps, when there is a differ- 
ence of 5 D. between the two meridians, the one 
figure will exactly overlap the other : for higher 
degrees we have to calculate how much the figure 
with the steps projects beyond the quadrilateral 
figure ; or we may place in the tube a stronger bi- 
refracting prism, then each step may be counted as 
two dioptres instead of one. 

Nordenson has obtained some interesting statistics 
with this ophthalmometer ( f Ophthalmic Review ' for 
July, 1883) in 226 school children. As a result of 
these statistics he is of opinion — 

1st. That the correction of corneal astigmatism by 
means of the lens in young persons is the rule. 

2nd. That corneal astigmatism amounting to one 
and a half dioptres is incompatible with normal 
acuteness of vision. 

Nordenson's observations agree with the opinion 
expressed by Javal that astigmatism predisposes to 
myopia. 

Tweedy's Optometer affords an easy method of 
estimating the refraction in astigmatism. It consists 
essentially of a plate carrying the figure of a dial 
marked with fine dark radiating lines at angles of 15° 
with each other ; the plate is attached to a horizontal 
bar half a metre long, divided into centimetres, on 
which it may be made to slide: at the proximal end 
of the bar is a semicircular clip, marked with degrees 



ASTIGMATISM 



17! 



corresponding to those on the dial, and intended to 
hold the cylindrical lens. In order to use the instru- 
ment properly, the following instructions must be 
strictly complied with : 

1st. The eye about to be examined haying pre- 
viously been placed completely under atropine, and 
made artificially myopic to about 4 D. by means of a 
Fig. 88. 




strong conyex lens placed in a spectacle frame, and 
the opposite eye excluded by an opaque disc, the 
patient should sit down before the instrument, place 
the eye with the lens before it close to the clip, and 
with the head erect should look straight in front at 
the radiating lines of the dial. 

2nd. The dial haying first been removed beyond 
the point of distinct vision, should then be gradually 



180 THE REFRACTION OF THE EYE 

approximated along the bar, until at least one of the 
lines is clearly and distinctly seen ; after this the dial 
should on no account be moved, but its distance from 
the eye accurately noted. 

If all the radiating lines come into view with equal 
clearness at the same time there is but slight astig- 
matism; but if whilst one line is clearly seen, that at 
right angles to it is blurred, there is astigmatism, 
which may be corrected by placing in the semicir- 
cular clip a concave cylindrical lens with its axis 
parallel to the blurred line, or at right angles to that 
first distinctly seen. 

From the result of (2) we learn (a) the direction of 
the two principal meridians, of maximum and mini- 
mum refraction; (b) the presence or absence of 
hypermetropia or myopia, and the degree; (c) the 
presence or absence of abnormal regular astigmatism, 
including its direction and degree, (a) The meridian 
of greatest refraction is parallel to the line seen at 
the greatest distance of distinct vision, while the 
meridian of least refraction is always at right angles 
to it. (b) The presence or absence of ametropia is 
determined by the distance at which the radiating 
lines are clearly seen. If there be emmetropia, the 
lines will be seen exactly at the distance of the focal 
length of the lens employed to produce the artificial 
myopia : if there be hypermetropia, the lines will be 
seen beyond that point : if myopia, within. The ic _ 
of ametropia may be estimated by the following cal- 
culation. The greatest distance of distinct vision, 
minus the focal length of the lens, divided 



ASTIGMATISM 181 

the multiple of these numbers, equals the degree of 
ametropia. 

(c) If, however, there be astigmatism, the above 
calculation will give the refraction for the meridian 
of least refraction only; the degree of astigmatism 
will be represented by the focal length of the weakest 
concave cylinder, which, placed with its axis parallel 
to the blurred line, makes this line as clear and 
distinct as that first seen. The whole ametropia may 
then be corrected by combining the spherical lens 
required for the correction of the meridian of least 
refraction, with the weakest cylindrical lens which 
by actual experimentation has been found sufficient 
to correct the astigmatism. 

Placido's disc consists of a circular sheet of tin on 
which is painted concentric circles of black and white ; 
it enables one to detect the chief meridians of the 
cornea at a glance. The patient being placed with 
his back to the light is directed to look at the centre 
of the disc, while the observer, holding the instru- 
ment close to his own eye and at a convenient dis- 
tance from the patient's, looks through the hole in its 
centre; he sees an image of the concentric circles 
reflected on the cornea : if astigmatism exist, the 
rings will appear elliptical, with the long' axis corre- 
sponding with the meridian of least curvature. Cases 
of irregular astigmatism and conical cornea are easily 
detected by this method. 

The stenopaic slit, which consists of a metal disc 
having an oblong opening in it about 2 mm. broad, 
is used by some observers for working out cases of 



182 THE REFRACTION OP THE EYE 

astigmatism. The disc is placed in a trial frame in 
front of the eye we wish to examine ; and while the 
patient looks steadily at the distant type the disc is 
slowly rotated, so that the slit is brought successively 
in front of each meridian, the position in which the 
best vision is obtained is noted; we then try convex 
and concave spheres in front of the slit, to see if any 
improvement take place. The slit is now in line with 
one of the chief meridians; let us turn the disc round 
90°, so that the slit may occupy the position of the 
other chief meridian, and find out what sphere most 
improves vision. Thus, supposing with the slit in the 
vertical direction the patient reads -J, while convex 
glasses in front of the slit make it indistinct, the 
vertical meridian is emmetropic ; and on turning the 
slit so that it is horizontal, the patient reads -f-^, but 
with + 2D. sphere the vision equals |-, the horizontal 
meridian is then hypermetropic, and the case is there- 
fore one of simple hypermetropic astigmatism, re- 
quiring for its correction + 2D. cylinder axis vertical. 
On looking through the slit, placed between the prin- 
cipal meridians, circles of diffusion are formed, and 
the object has the appearance of being drawn out in 
the direction of the slit. 

Dr. Tempest Anderson, of York, has invented an 
ingenious instrument by which astigmatism may 
be estimated in a subjective manner; an image of 
an illuminated radiating screen is thrown on the 
retina, and is visible to the observer; the position 
of the screen on a graduated bar shows the refrac- 
tion. 



ASTIGMATISM 183 

The inventor claims for his instrnment the follow- 
ing advantages : 

1. The observations and measurements are made by 
the observer, and are entirely independent of the 
patient's sensations, though these may be used as an 
adjunct if wished. 

2. An image thrown on the retina being used as an 
object, the error arising from the vessels or optic 
nerve being before or behind the retina is avoided. 

3. The refraction and accommodation of the ob- 
server does not affect the result. It is only necessary 
that he should be able to see whether certain lines 
are sharply defined. 

In addition to the methods already described for 
estimating astigmatism, many others are known. 

See Cases 3, 4, 5, 6, 7, 8, 9, p. 110, &c; also 20 
and 21, p. 254. 



1S4 THE REFRACTION OF THE EYE 



Anisometropia 

Anisometropia (a, priv. ; 7<roc, equal ; /utrpov, mea- 
sure ; w\p, tlie eve) is the term applied to eases which 
frequently occur, where the two eyes vary in their 
refraction. The defect is usually congenital, but it 
may be acquired, as in aphakia or loss of accommoda- 
tion in one eye. Every possible combination may 
exist : one eye may be emmetropic, the other myopic 
or hypermetropic ; or one more myopic, hyperme- 
tropic, or astigmatic than the other. 

Anisometropia may be met with under three chief 
forms : 

1. Cases where binocular vision is present. 

2. When the eyes are used alternately. 

3. One eye is permanently excluded from vision. 

1. In the first variety the difference in refraction is 
usually not very great ; and if it were possible for the 
patient to accommodate unequally in the two eyes, he 
might be able to obtain clear images on each retina ; 
but it is probable that the two ciliary muscles make 
the same effort, with the result that in one eye the 
image is well defined, in the other indistinct. 

2. When the eyes are used alternately, then one 
eye is usually emmetropic or hypermetropic, and is 
employed for distant vision : while the other is myopic 
and is used for near work. 

3. When the difference between the two eyes is very 
great the best eye may be used exclusively, while 
the vision in the other is very bad, and frequently 



ANISOMETROPIA 185 

deviates outwards or inwards ; in many of these cases 
one eye is emmetropic or slightly hypermetropic, the 
other highly myopic. 

Treatment. — When the difference is not very great 
(1 or 1*5 D.), and vision in both eyes is good, we may 
give each eye its correction for constant use : for so 
long as the eye whose refraction is the more defective 
still co-operates in binocular vision, sight is improved 
thereby. Especially is this full correction useful in 
cases of myopia with divergent strabismus, the in- 
creased stimulus to binocular vision being sometimes 
sufficient to prevent the squint. 

Many cases do not stand their full correction for 
each eye with comfort ; they complain of strain, dis- 
comfort, and headache, though the younger the patient 
the less liable is he to suffer from these symptoms. 

The asthenopia which often results from giving 
each eye its exact correction may possibly be due to 
the different prismatic effect which must result when 
the patient looks obliquely through his two glasses 
which have a different refractive power, and it has 
been suggested by Mr. W. A. Dixey to overcome this 
difficulty by using a bifocal lens before the eye 
whose refraction is more defective. This can be done 
by grinding a small central portion of the glass — that, 
in fact, which is immediately in front of the pupil — 
of such a focus as to fully correct the error, while the 
other part of the lens will be ground of the same 
focus as the glass in front of the less defective eye. 
Thus, to take an example, a patient has 4 D. of myopia 
in the right eye and 2 D. in the left ; for the right eye 



186 THE REFRACTION OF THE EYE 

he would require a glass which was — 4 D. in the 
centre, — 2 D. at the margin, while the left eye would 
be supplied with — 2D. 

When one eye is emmetropic and the other 
myopic, no glass will probably be required, the 
emmetropic eye being used for distance, the myopic 
eye for reading, &c. When the difference in the 
refraction is greater than 1*5 D. we may have to 
be satisfied with partially correcting the difference, 
and this result can only be arrived at by trying 
each case, some people tolerating a much fuller 
correction than others, our object being to give as 
near as possible the full correction for each eye. 
When binocular vision does not exist, frequently no 
attempt can be made to correct the two eyes, and 
then we generally give glasses that suit the best eye. 
In cases of aphakia, &c. } where one eye is used almost 
entirely, while the other, though defective, still pos- 
sesses vision, it is an excellent plan to insist on the 
latter being daily exercised with a suitable glass, 
the good eve being at the same time covered; by this 
means the bad eye is prevented from becoming worse, 
and can at any time be utilised should occasion 
require. 

See Cases 14 and 15, pp. 248 and 251. 



PRESBYOPIA 187 



CHAPTER IX 

presbyopia. Pr. (7rp£(r(Dvg, old; wip, eye) 

With advancing age many changes take place in 
the eye. The acuteness of vision becomes less, owing 
partly to a loss of transparency in the media, and 
partly to a diminntion in the perceptive and con- 
ductive powers of the retina and the optic nerve. At 
the age of forty the acuteness of vision is almost 
unaltered, the bottom line of the distant type being 
read at a little over 6 metres ; at fifty it can still be 
read at 6 metres, but after this time it diminishes 
regularly, so that by the eightieth year vision may 
have decreased to -f or -fy. In addition to these 
changes, the accommodation gradually diminishes 
from a very early period, the near point slowly but 
steadily receding. This change in the accommo- 
dation occurs in all eyes, whatever their refraction, 
and is due to an increased firmness of the lens, 
whereby its elasticity is lessened; and perhaps also 
in some slight degree to loss of power in the ciliary 
muscle due to advancing age. The lens also 
approaches the cornea, and becomes somewhat 
flatter. This failure of the accommodation begins 
as early as the tenth year, at an age when all the 
functions of the body are still developing. 



188 THE REFRACTION OF THE EYE 

Presbyopia must therefore be looked upon as a 
physiological condition. 

When the binocular near point has receded beyond 
the distance at which we are accustomed to read 

Fig. 89. 



Diagram Blowing the course of accommodation in an emme- 
tropic eye. The figures at the top of the diagram indicate 
the age; those at the side the amount of accommodation 
and the p. p. in centimetres; the oblique line represents 
the course of the punctum proximum, and the horizontal 
line that of the punctum remotum ; the space between the 
two lines gives the amplitude of accommodation. From 
this diagram we can calculate the amplitude of accommo- 
dation possessed at any age. 

and write with comfort, we become restricted in our 
work. Donders has fixed this point at 22 cm. 



PRESBYOPIA 189 

Presbyopia, therefore, may be arbitrarily stated to 
exist when the binocular near point has receded to 22 
cm., and this occurs usually in the emmetrope about 
the age of forty-five. Because in order to see at 22 
cm., a positive refractive power of 4'5 is necessary 
(Vr =4*5); at the age of forty, the eye possesses 
just this amount of refi active power; but if the eye 
has not so much accommodation, then we must give 
such a convex glass which, added to it, brings up 
the positive refraction to 4*5 D. : for example, at 
the age of fifty-five, when the eye 'possesses only 
1*5 D. of accommodation, we give a convex glass 
of 3 D., because 1*5 D. + 3 D. = 4*5 D. (see table, 
p. 191). 

To find the punctum proximum of an emmetrope, 
we have only to divide the number of dioptres of 
accommodation which he possesses into 100 cm. 
Thus at twenty there are 10 D. of accommodation; 
this would give us 10 cm. as the near point. At 
forty there are 4*5 D., in which case the near point is 
22 cm. 

When the punctum proximum has receded to 22 
cm., the point at which it is convenient to read is 
considerably further away, since for sustained vision 
only about half of the accommodation can be used. 
Thus a person with 4 D. of accommodation would 
have his near point at 25 cm. with the maximum 
contraction of his ciliary muscle, and if he can only 
comfortably use about half this for continuous work, 
his reading point would be 50 cm. ; this is too great 
a distance. We bring back the near point by convex 



190 THE REFRACTION OF THE EYE 

glasses, which is practically the same as increasing 
the accommodation. 

Although we have said that only about one half of 
the accommodation can be used for sustained vision, 
this is not absolutely correct : the amount which must 
be in reserve varies much with different individuals; 
thus in one case with a surplus of 1 D. much work 
can be done, whereas in another a surplus of 3 or 4 D. 
is necessary. 

Symptoms. — The presbyope sees well at a distance, 
but has difficulty in maintaining clear vision for near 
objects ; the chief symptoms are a feeling of weari- 
ness in the eyes after reading, especially in the 
evenings, small objects being less easily seen than 
formerly, because, having to be held further from the 
eyes, they subtend a smaller visual angle. The patient 
seeks a strong light, or places the lamp he is using 
between his eye and the book; by doing this he 
causes his pupils to contract, and so lessens his circles 
of diffusion; he avoids small print, and holds the 
book or work further away. These symptoms are 
due to a recession of the near point, and if asthe- 
nopia occur, this may be dependent upon a disturbance 
of the balance between accommodation and conver- 
gence ; the convergence being the same for any given 
point, a much greater accommodative effort is neces- 
sary than was formerly the case. 

The treatment of presbyopia consists in prescribing 
convex spectacles for reading and near work, so as to 
bring back the near point to a convenient distance. 
The best reading distance for a person with normal 



PRESBYOPIA 



191 



visual acuity is from 30 to 40 cm. ; most emmetropes 
will, therefore, require a convex glass for near work 
soon after the age of forty-five ; we have only to re- 
member to add on + 1 D. for every five years until 
we have reached + 3*5 D. 

An emmetrope with good visual acuity will never re- 
quire a stronger glass than + 3*5 D. even when sixty or 
seventy years of age, because these glasses adapt him 
for a distance of 30 cm. without any accommodation. 

Should the patient have defective vision, then it 
may be necessary for him to hold near objects much 
closer to the eyes than 30 cm. in order that he may 
get larger retinal images ; here we should be justified 
in prescribing much stronger glasses. 

The following table gives approximately the 
strength of glasses required by emmetropes at dif- 
ferent ages to bring back their punctum proximum 
to 22 cm. : 



Age. 
45 


Amount of accommodation 

possessed at that 

age. 

35 D. 


The near 
point. 

28 cm. 


Glass required to 

bring back 

p. p. to 22 cm. 

+ 1D. 


50 


2-5 D. 


40 cm. 


+ 2D. 


55 


1-5 D. 


67 cm. 


+ 3D. 


60 


•5D. 


200 cm. 


+ 4D. 


70 


•0D. 


infinity 


+ 4-5 D. 



It must be understood that this table indicates the 
glass that brings the p. p. back to 22 cm., and if we 
wish the patient to read at 33 or 40 cm. these glasses 
will in practice be found too strong. 

To find the glass required in presbyopia, we sub- 
stract the glass which represents the receded near 
point, from the glass whose focus represents the point 



192 THR REFRACTION OP THE EYE 

we wish to make the near point. Thus the near point 
has receded to 50 cm. ; the glass representing this 
point is + 2 D. (W° = 2). We wish to bring the 
near point to 25 cm. ; this would be -f 4 D. (yy> = 4) ; 
hence 4- 2 D. from + 4 D. gives + 2 D. as the glass 
required. 

Although glasses can be frequently thus ordered 
by a sort of rule of thumb, it is always well to bear 
in mind that the definition given of presbyopic with 
reference to its near point is entirely an arbitrary one, 
and that we must take into account the distance at 
which the individual has been accustomed to read and 
work. In this there is great variety. Many small 
people wurk and read at 25 cm., whereas very tall 
people may be uncomfortable unless the book they are 
reading is 35 or 40 cm. away. The distance for which 
the presbyope requires spectacles will also vary much 
according to the occupation for which he requires 
them; thus a carpenter sixty years old with emme- 
tropia may require to work at his bench, which may be 
at one metre away ; to enable him to see at this distance 
he will require -f 1 D., while for reading at 33 cm. 
he will require -i- 3D. 

There exists a popular prejudice against the use of 
strong glasses, all sorts of maladies having been attri- 
buted to their use. This prejudice is quite unfounded; 
if the lenses are too strong they may bring the reading 
point inconveniently near, and so produce asthenopia 
by causing the patient to converge excessively. 

Before ordering glasses for presbyopia, always 
try the patient's distant vision, so that any hyper- 



PRESBYOPIA 193 

metropia or myopia may be recognised. If hyper- 
metropia exist, the amount must be added to 
the presbyopic glass; if myopia, it must be sub- 
tracted. Thus a patient with hypermetropia requir- 
ing + 2 D. for its correction, at the age of forty-five 
will require + 3 D. for reading (H. 2D. + Pr. 1 D. 
= + 3D.). 

A myope of 1 D. will require no glass at the age of 
forty-five (M. 1 D. + Pr. 1 D. = 0). If the myopia 
be 3*5 D., then the patient can never require a glass 
for presbyopia, his far point being 30 cm. always. 
His near point may recede to this distance when all 
accommodation is lost, but he will still be able to read 
at that distance, though at that distance only. 

But allowance must be made for the fact that as 
age advances the refraction of the eye diminishes ; 
in other words, the eye if emmetropic becomes hyper- 
metropic (called acquired hypermetropia). The my- 
opic eye becomes less myopic, so that a real improve- 
ment in vision takes place. The hypermetropic eye 
becomes more hypermetropic. This change takes 
place at a regular rate in all eyes ; at fifty-five the 
refraction has diminished *25 D., at sixty-five '75 D., 
at sixty-eight 1 D., and at eighty as much as 2*5 D. 
Thus at eighty an emmetrope will have acquired 
2*5 D. of hypermetropia, and will therefore require a 
convex glass + 2*5 D. for distant objects to be seen 
clearly. A myope of 2*5 D. would at eighty have be- 
come emmetropic, and require no glass for distance. A 
hypermetrope of 2*5 D. will add on to his defect 2*5 D., 
and will require a 4- 5 D. for distance. This change 

13 



194 THE EFFRACTION OF THE EYF 

is due to sclerosis and enlargement of the crystalline 
lens, by which its refractive power is diminished. 

Dr. Scheffler some years ago proposed the use of 
what he called orthoscopic lenses, that is, lenses with 
two elements, a sphere and a prism, so proportioned 
that the amount of accommodation and convergence 
should exactly correspond. Thus in the case of a 
presbyope aged fifty, requiring + 2 D. to make him 
read comfortably at 25 cm., it would be combined 
with a prism of 2 m.a. base inwards, the result being 
that the patient would then have to put on 2 D. of 
his accommodation and 2 m.a. of convergence, and 
thus these two functions would be used in equal 
degrees. The results, however, are not so good as 
might have been hoped; the glasses are too heavy, 
and on looking at a flat surface some distortion is 
produced. Nevertheless cases do occur in which, 
though the presbyopia is corrected, the patient after 
reading a short time complains of asthenopia. Such 
cases are frequently at once and completely relieved 
by combining with the spheres, prisms of 2° or 3° 
with their bases inwards; or by having the lenses 
decentred, forming convex prismo-spheres (Fig. 101). 

A lens of 1 D. must be decentred 8*7 mm. to pro- 
duce a prismatic effect of 1°. Thus in order to find 
out the amount that a lens should be decentred 
to produce a given prismatic effect, it is necessary to 
multiply 8*7 by the number of the prisms we wish to 
use, and divide the result by the number of the 
lens. Thus, to take an example, with a + 6 D. we 
wish to produce a prismatic effect of 2°, then 



PARALYSIS OF THE ACCOMMODATION 195 

7x2 17-4 



2'9 mm. ; the glasses would require 



to be decentred inwards 2*9 mm. 

Care must be taken to see that the glasses are 
properly centred, unless they have been ordered 
otherwise ; for if the frames are too broad, the pris- 
matic effect produced is very apt to give rise to 
asthenopia, by disturbing the relations between con- 
vergence and accommodation. 

In cases where the convex glasses have frequently 
to be changed for stronger ones, "glaucoma" should 
be carefully looked for; and if any symptoms of 
it appear, no near work must be allowed, as it is 
important to avoid all possible causes of tension. 

The commencement of cataract may in some cases 
hasten presbyopia, but it more frequently produces 
myopia, so that the presbyope requires his glasses 
diminished in strength. 

In each case of presbyopia first test the patient's 
distant vision, so as to detect any hypermetropia, 
myopia, or astigmatism ; and having recorded this, we 
add the glass which he requires for his presbyopia 
and try him with the reading type : should they suit, 
we direct the patient to read with them for half an 
hour or so ; if found satisfactory we order spectacles 
of this strength. 

See Cases 12, 16, 17, and 18, p. 247, &c. 

Paralysis op the Accommodation 

Paralysis of the accommodation, either partial or 
complete, arises from loss of power in the ciliary 



196 THE REFRACTION OP THE EYE 

muscle (cycloplegia), and is due to paralysis of the 
third nerve, or of that branch of it which supplies the 
muscle of accommodation and the circular fibres of 
the iris. Cases do occasionally occur, though very 
rarely, of paralysis of the ciliary muscle not involving 
the constrictor pupillag. Both eyes may be affected, 
or only one. 

When the paralysis is confined to the ciliary muscle 
and iris, it goes by the name of ophthalmoplegia 
interna. 

Causes. — Atropine is the most common cause, but it 
may be due to diphtheria, rheumatism, fever, any 
complaint of a lowering character, cerebral trouble, 
syphilis, diabetes, or some reflex irritation, e.g. decayed 
teeth, &c. ; the cause may, however, not be apparent. 
When the whole third nerve is involved, ptosis, ex- 
ternal strabismus, &c, occur; but in those cases 
where the branch supplying the ciliary muscle and 
the circular fibres of the iris is alone implicated, the 
indicating symptoms are asthenopia, dilatation of- the 
pupil, and loss of the power of accommodation, 
whereby the patient, though able to see distant 
objects well (if emmetropic), is unable to read or do 
any near work. If hypermetropic, both near and 
distant vision will be impaired ; if myopic, he is able 
to see only at his far point. We try the patient at 
the distant type, and if he is able to see f and yet is 
not able to read near type, the diagnosis is obvious. 

Treatment consists in giving such convex glasses as 
enable him to read. In order to bring the emme- 
trope's far point from infinity to 33 cm., 4- 3 D. is 



SPASM OF THE ACCOMMODATION 197 

required (^ = 3). We must bear in mind that 
by encouraging the action of the ciliary muscle we 
hasten the patient's recovery ; we therefore order the 
weakest convex glasses with which he is able to read, 
changing them for weaker ones occasionally as the 
ciliary muscle gains strength. Sulphate of eserine in 
solution, gr. j to $], causes contraction of the ciliary 
muscle as well as of the iris, and temporarily relieves 
the symptoms. I think much good sometimes results 
from its use once every other day for some weeks; 
the ciliary muscle being made to contract, relaxing 
again as the effect of the myotic passes off : some- 
times the local application of electricity is useful. 
Attention must be paid to the general health, iodide 
of potassium or nerve-tonics being given when indi- 
cated by the cause. 
See Case 13, p. 248. 



Spasm op the Accommodation 

Spasm of the accommodation may be of two kinds, 
Tonic and Clonic. 

Clonic spasm occurs only when the eye is in use, 
ceasing as soon as it is in a condition of repose. 

Tonic spasm is more permanent, requiring atropine 
or one of the other mydriatics for its relief ; the ex- 
pression spasm of accommodation usually refers to this 
variety of the disorder. 

Tonic spasm of the ciliary muscle may be occasion- 
ally met with in eyes whatever their refraction, 



198 THE REFRACTION OF THE EYE 

though most commonly in cases of hypermetropia and 
low myopia; it has the effect of increasing the re- 
fraction of the eye, and is found most frequently in 
children. 

Causes. — It may occur as a result of uncorrected 
ametropia, or in emmetropia from overwork, espe- 
cially when such work has been done in a bad light ; 
as a result of contusion of the eyeball, and sometimes 
it occurs with cyclitis. 

Symptoms. — It usually affects both eyes, giving rise 
to symptoms of asthenopia with a feeling of con- 
striction and discomfort in the eyes themselves ; there 
may be an increased secretion of tears with or without 
blepharospasm; the acuteness of vision may be 
diminished and is very variable, while the size of the 
pupil usually remains unaffected. In emmetropia we 
may get symptoms of myopia, owing to the parallel 
rays coming to a focus in front of the retina. In 
hypermetropia the symptoms may also simulate 
myopia, and for this we should always be on our 
guard. I have on several occasions seen hyper- 
metropes going about wearing concave glasses to 
correct their supposed short-sightedness. Some 
time ago I saw a young man who had worn — 7 D. 
constantly for years, though his refraction was really 
emmetropic. In myopia the real defect is apparently 
increased, and we might be in danger of ordering too 
strong concave glasses, &c. For these reasons the 
.matic use of atropine in young people (whereby 
one is enabled to estimate and record the exact state 
of the refraction) cannot be too strongly insisted upon. 



SPASM OF THE ACCOMMODATION 199 

The treatment, where spasm of the ciliary muscle is 
suspected, is to drop into the eyes three times a day 
a solution of sulphate of atropine, grs. iv to Jj, for 
two or three weeks ; this quickly relieves the spasm, 
and gives the eyes complete rest : any ametropia that 
may exist must be corrected, and the patient's 
general health attended to, tonics being administered 
if necessary. 

A few cases of acute spasm of the accommodation 
have been recorded which resisted the treatment by 
atropine ; the spasm, though relaxed by this means, 
returned as soon as the atropine was discontinued. 

See Case 1, p. 110. 



200 THE REFRACTION OP THE EYE 



CHAPTER X 

strabismus [oT/of^w, I turn aside] 

Strabismus exists when there is a deviation in the 
direction of the eyes, so that the visual axes are not 
directed to the same object. 

Strabismus may be divided into two classes — 
Concomitant. 
Paralytic. 

Concomitant strabismus is a frequent result and 
complication of refractive errors, and has already been 
referred to on pages 124 and 146. 

The deviation of one eye from its correct position 
is the result of disturbed muscular equilibrium, and 
may be due to — 

1. Defective anatomical conditions, or 

2. Abnormal innervation causing contraction of 

the muscles not in accordance with the re- 
quirements of binocular vision. Most cases 
of concomitant strabismus are due to this 
second cause. 
The points to note when a case of strabismus 
presents itself are — 

1. Is the strabismus real or only apparent ? 



STEABISMUS 201 

2. If real, to which variety does it belong ? 

3. Which is the deviating eye ? 

4. In which direction is the deviation ? 

5. What is the degree of the deviation ? 

6. What is the cause of the strabismus ? 

The first of these questions may seem unnecessary, 
but it is not always easy to say if squint is really 
present or not, because one judges of the direction of 
the eyes by the position of the centres of the cornea?, 
or rather by the optic axes, and these may diverge 
slightly while the visual axes are really parallel. This 
requires some explanation. 

The visual axis is the line passing from the macula 
through the nodal point to the object looked at, shown 
in the following figures asjv m. 

Fig. 90. 




The optic axis is the line passing through the nodal 
point and the centre of the cornea to the inner side 
of the macular, o h in figures. 

It will thus be seen that these two axes form an 
angle at the nodal point which in emmetropia amounts 
usually to 5° (Fig. 90) . 

This is called the angle a, and when thus formed 



202 



THE REFRACTION OP THE EYE 



by the crossing of the visual and optic axes it is said 
to be positive. 

In hypennetropia (Fig. 91) the angle a increases 



Fig. 91. 




with the degree of hypermetropiaj and if it be high 
may attain 7°, 8°, or even more; this large angle 
gives to the eye an appearance of divergence. 



Fig. 92. 




xr. The macula. K. The nodal point, b. Optic nerve. v.'The 
object. T m. The visual axis, o ir. The optic axis. a. 
The angle alpha formed between the visual and optic axes. 
C. The centre of rotation of the eyeball situated on the 
optic axis. y. The angle gamma (Fig. 90) is formed at 
the centre of rotation of the eyeball, by the optic axis and 
a line drawn from the centre to the object looked at. 

Iii myopia (Fig. 92) the angle a decreases, and in 
high myopia the visual axis may approach the optic 



STRABISMUS 203 

axis, so that the angle a is very small, or it may 
coincide with it when no angle is formed ; or even be 
altogether on the outer side of it, when the angle is 
said to be negative. This small angle a gives to the 
eyes an appearance of convergence.* 

In order to find out the variety to which our case 
of strabismus belongs, as well as to decide which is 
the deviating eye, we direct the patient to look at an 
object held about a metre in front of him, then 
gradually bring this object nearer to him, so as to call 
into action the accommodation : if both visual axes 
continue to be directed steadily towards the object 
as it is made to approach the eyes, the case is one 
of apparent strabismus; but if one eye fix the object 
while the other, after following it up to a certain dis- 
tance, suddenly deviates inwards or outwards, the 
condition is spoken of as concomitant strabismus 
(convergent or divergent) ; or both eyes may follow 
the object up to a certain point, when one stops, 
after perhaps making a few jerking oscillating move- 
ments; it then belongs to the paralytic variety of 
strabismus. 

Again, direct the patient to look at some object held 
midway between the two eyes and about a metre 
away; if the right eye fix the object while the left 
deviates inward, we mark upon the edge of the lower 
lids with a pen the position of the external margin of 

* Another angle sometimes mentioned is the angle y, which is the 
angle formed at the centre of rotation of the eye by the optic axis 
and a line drawn from this centre to the ohject looked at, shown in 
Fig. 90. 



204 



THE REFRACTION OP THE EYE 



the cornea in each eye, s and p : on covering the 
right eye with a card, the left will at once make a 



Fig. 93. 




S D 




L 





D 



The two upper diagrams, e and L, show the primary position 
of the eyes, the right heing the fixing eye, while the left is 
deviating inwards. On covering the right eye with a card, 
as shown by the dotted lines, the left eye fixes while the 
right deviates inwards. P d therefore indicates the primary 
deviation, s D the secondary deviation. 

movement outwards to fix the object, and we make a 
second ink mark, d, on the lids corresponding with 
the outer edges of the cornea in this position. 

The distance p d gives us the amount of frimary 
deviation ; it may further be seen that on covering the 
right eye so as to cause the left eye to fix the object 
looked at, the right eye has made a movement inwards 
behind the screen, a secondary deviation has taken 



STRABISMUS 205 

place, this is recorded by an ink mark on the right 
lower lid at d ; we have thus found the linear measure- 
ment of the secondary deviation, s d. 

The primary deviation (p d) will be found to 
equal the secondary deviation (s d), a characteristic 
of concomitant squint. 

In paralytic squint the secondary deviation is always 
greater than the primary. 

This variety of squint is known as concomitant, 
because the squinting eye follows the fixing one in all 
its movements, the amount of squint always being the 
same whatever be the direction in which the eyes are 
turned; therefore the range of movement in con- 
comitant squint is as great as in cases where no squint 
exists ; it is simply displaced. In the paralytic form, 
the movements of the squinting eye are usually much 
curtailed ; this we easily detect by holding up the 
finger about 50 cm. in front of the patient, and 
directing him, while keeping the head still, to follow 
the movements of the finger, which is moved to either 
side, then up and down. 

So that in concomitant strabismus the squinting 
eye will almost exactly accompany the other, the 
visual lines being at the same angle, except perhaps 
in the extreme periphery, whereas in the paralytic 
variety one eye will stop at a certain point, while the 
other eye continues to follow the finger. 

Concomitant squint is characterised by the fact 
that the primary and secondary deviations are equal ; 
in paralytic strabismus the secondary deviation ex- 
ceeds the primary ; in paralytic squint the diplopia is 



206 



THE REFRACTION OF THE EYE 



the most prominent subjective symptom, while in the 
concomitant variety it is seldom complained of. 

When either eye fixes indifferently, the vision 
being equally good in both, the strabismus is alternat- 
ing ; when the same eye always squints, it is mono- 
lateral or constant. The vision in the squinting eye is 
often below that in the fixing one. 

Periodic is the name applied to the squint when it 
only comes on occasionally, as after looking for some 
time at near objects. With judicious treatment this 
variety can be cured without operation ; if neglected 
it generally passes on into one of the constant forms. 

There are several ways by which the amount of the 
deviation may be estimated. Thus we may record it 
in the form of a diagram (Fig. 93). 

The strabismometer (Fig. 94) consists of a handle 



Fig. 94. 




Sti'iibisinoineter, 



STRABISMUS 207 

supporting a small ivory plate, shaped to the lower 
lid, and having on it a scale by which we measure the 
amount of deviation of the centre of the pupil. This 
is an easy method of measuring the strabismus, but is 
not to be depended upon. 

The measurement of the angle of the strabismus is 
the only reliable and exact method of recording the 
amount of squint, and is the method therefore recom- 
mended. The angle of the strabismus may be defined 
as that angle which the visual axis makes with the 
direction it should have in a normal state. 

For this measurement we require a perimeter, in 
front of which we seat the patient, with the quadrant 
placed according to the kind of squint we are about 
to measure; if it be convergent or divergent, then 
the quadrant is placed horizontally. The patient 
being seated so that his deviating eye is in the centre 
of the instrument, we direct him to fix with both eyes 
some distant object (o, Fig. 95) placed in a line with 
the centre of the perimeter; a lighted candle is 
moved gradually along the inside of the quadrant 
from the centre of the instrument outwards; the 
observer, following the movement of the candle with 
his head, stops as soon as the reflection of the candle 
on the cornea of the squinting eye occupies the centre 
of its pupil, this gives the direction of the optic axis ; 
what we really wanted was the direction of the visual 
axis, but for all practical purposes the former is suf- 
ficient. The degree is read off the quadrant at the 
point where the candle was stopped, and this result 
recorded. The angle of deviation for near vision 



208 THE EFFRACTION OF THE EYK 

should next be taken; this may be done by requesting 
the patient to look at the centre of the perimeter, 

Fig. 95.* 




proceeding with the candle as before, and recording 
the result. 

Concomitant strabismus is intimately connected 
with hypermetropia and myopia ; it may be — 
Convergent 

or 
Divergent. 
Convergent Concomitant Strabismus. — On looking at 
any object, one eye only is directed to it; the other, 

* In the above diagram, o is intended to represent a distant object; 
it is placed near the perimeter in order to take up less room. 



CONVERGENT STRABISMUS 209 

as the name implies, turns inwards so that the angle 
of convergence is much greater in the deviating than 
in the fixing eye. 

The common cause of this variety of squint is 
hypermetropia, at least 80 per cent, of the cases are 
due to this cause ; its method of production depends 
upon the intimate connection that exists between 
accommodation and convergence. 

The convergence is most marked when looking at 
near objects ; sometimes there may be no squint when 
distant objects are viewed. 

A person who is hypermetropic requires to use 
some of his accommodation for distance; for near 
objects he must, of course, use still more, and for 
every increase in the accommodation there is a desire 
for an equal increase in the degree of convergence. 
Thus an emmetropic individual, accommodating for 
an object at 30 cm., would at the same time converge 
for that particular point. 

If the individual were hypermetropic to the extent 
of 4 D., and supposing the amplitude of his accommo- 
dation amounted to 8 D. ; then he would require to 
use half this (4 D.) to enable him to bring* parallel 
rays to a focus on the retina ; and he would have the 
tendency at the same time to use 4 metre-angles of 
convergence. Thus for distant objects he would have 
an inclination to converge, his internal recti acting*; 
and it is only by the increased tension of the external 
recti, called into action by the desire which all eyes 
possess for singleness of vision, that convergence is 
prevented. The more we accommodate the greater 

14 



210 



THE EEFRACT10N OF THE EYE 



is the stimulus to converge, so that on looking at near 
objects — which necessitates an increase of the accom- 



Fig. 96. 







E. Right eye directed to object o. l. Left eye deviating 
inwards, m. Macula. 

modation — an increased tendency to convergence is 
produced. 

Now, if the hypermetropia be of such a degree 
that for any given point of convergence it exceeds 
the positive part of the relative accommodation (Fig. 
34, p. 50), one of two things must occur; either the 
patient must see indistinctly by not accommodating 



CONVERGENT STRABISMUS 211 

sufficiently, or one eye must be allowed to converge. 
Some patients will prefer binocular indistinct vision; 
other s, single clear vision with squint. 

One occasionally finds an individual who can thus 
choose which he will do ; we are trying his acuteness 
of vision at the distant type perhaps ; he stops at 
some place, we will suppose -^-, and says that he is 
unable to read the next two lines unless he squint. 
The accommodation necessary to read -§-, makes a 
heavier call on the convergence than can be borne ; 
such a case forms a good illustration of the manner 
in which convergent strabismus is produced in a 
hypermetrope. 

Hence, if the impulse to see distinctly is greater 
than the desire to retain binocular vision, one eye 
yields, and squint occurs ; at first diplopia follows 
the convergence, and is always in the opposite direc- 
tion to the deviation. Possibly the convergence of 
the deviating eye is increased by the desire that the 
weaker image may be made still weaker, by falling 
on a more peripheral part of the retina. At first the 
diplopia may be very annoying, but by degrees the 
sensorium learns to suppress the image of the 
weaker eye, which after a time becomes amblyopic. 
The earlier the age at which the squint appears, 
the sooner does the sight in the deviating eye thus 
deteriorate. 

Some observers deny that amblyopia is ever 
developed as a result of squint, but consider the 
amblyopia congenital, and therefore one of the com- 
bining causes of the strabismus. 



212 THE REFRACTION OP THE EYE 

There would, however, seem to be two distinct 
varieties of amblyopia met with in squint : in one the 
amblyopia is of a very high degree, the vision being 
reduced in some cases to mere hand reflex; this 
variety is incapable of improvement, and is probably 
congenital, or if not congenital it is the result of a 
want of development of the brain centres that preside 
over the vision of this eye. 

The second variety of amblyopia is the result of 
want of practice, and consists of an awkwardness and 
difficulty in using the eye ; frequently a patient will 
say that he is unable to see with the eye at all, and 
yet when encouraged to read with it and with the 
proper optical correction may succeed in reaching 
Y^ or even -^ ; here practice is essential, in the hope 
that binocular vision may be regained. 

In amblyopia it will often be found that the vision 
is best on the temporal part of the field, that part 
which is most used in peripheral vision. 

In high degrees of hypermetropia, when no amount 
of accommodation can make vision distinct, squint is 
less likely to occur. It is usually, therefore, in cases 
of from 2 to 4 D. that convergent strabismus is 
most frequently met with, and it generally makes its 
appearance about the fourth or fifth year, — so soon, 
in fact, as the child begins to use its accommodation 
much for near objects. 

Anxious parents frequently have all sorts of excel- 
lent reasons to which they attribute the defect; they 
say that the child has been imitating its playmate, or 
that the nurse did something which caused it to 



CONVERGENT STRABISMUS 213 

squint, perhaps by making the child look too much, 
or too constantly in one direction. 

Any cause which by rendering the image in one 
eye less distinct than that in the other, as nebulae^ 
ulcers of the cornea, a difference in the refraction 
of the two eyes, or even wearing a shade for a few 
days for some trivial complaint, may, where hyper- 
metropia is present, combine to produce strabismus J 
the impulse for binocular vision is lessened, and the 
eye in which the fainter image is formed converges. 

It is thus seen that convergent strabismus gra- 
dually destroys binocular vision. In cases of hyper- 
metropia, where binocular vision does not exist owing 
to great difference in the refraction of the two eyes 
divergent strabismus may occasionally occur. 

This intimite connection between accommodation 
and convergence, together with the method of the 
production of strabismus, will be more easily under- 
stood by carrying out some such simple experiments 
as the following. We will assume the observer to be 
emmetropic : the strongest concave glass with which 
he, having binocular vision and being at a distance 
of six metres, can still read -§-, is the measure of the 
relative accommodation. The absolute accommodation 
is measured by the strongest concave glass with which 
each eye separately can read -|. In my own case, 
with -4D. before each eye, f can be seen singly 
and distinctly, - 4*5 D. renders it indistinct, and each 
increase in the glass increases the indistinctness, but 
produces no diplopia. Separately each eye can over- 
come — 7 D, Armed with -4D, before each eye. 



214 THE REFRACTION OF THE EYE 

I am able to see § distinctly, using, of course, 4 D. of 
my accommodation; if a coloured glass be placed 
before one eye, homonymous diplopia at once appears, 
proving that one eye has deviated inwards; with 

— 3 D. and the coloured glass, squint was produced, 
but with no weaker concave glass. 

On repeating the experiment in an individual 
with *5 D. of myopic astigmatism in the right eye, 
and emmetropia in the left, — 2D. before each eye 
was the strongest glass with which Jj- could be seen 
clearly and singly, — 2*5 D. did not render it indis- 
tinct, but produced diplopia. The absolute accom- 
modation for each eye amounted to 6 D. With 

— 2D. before each eye, the coloured glass was placed 
before the astigmatic one, and diplopia was pro- 
duced. With —ID. and the coloured glass the 
result was the same, except that the two images were 
nearer together. With — "5 D. actual diplopia was 
not produced. 

These experiments require but little explanation* 
In my own case, when using 4 D. of accommodation, 
there is a tendency also to use a corresponding 
amount of convergence ; I am conscious of this 
muscular disturbance by the effort I make, and by 
a feeling amounting almost to giddiness, produced 
when first looking through the — 4 D. The instin- 
tive desire to see clearly and singly is so great, that 
the external recti contract, thereby balancing the 
increased contraction of the internal recti. Any 
increase of my accommodation above 4 D. when 
looking at f causes the letters to become indistinct, 



DIVERGENT STRABISMUS 215 

the desire to maintain binocular vision being greater 
than that for clear images. On placing the coloured 
glass before one eye, we diminish the retinal impres- 
sion in that eye ; the demand for binocular vision is 
lessened, the external recti cease to act, and as a 
result of the increased action of the internal recti 
squint occurs. 

In the second experiment the retinal impression 
in one eye, even with so slight an amount of astigma- 
tism, is reduced so that with 2 D. of accommodation 
the desire for clear images is greater than that for 
binocular vision, and diplopia, the symptom of squint, 
appears. 

A certain number of cases of convergent strabismus 
get well with advancing age ; this is most likely to 
take place when the vision in both eyes is good, though 
it sometimes occurs even where a high degree of 
amblyopia is present and binocular vision cannot 
exist. An unsatisfactory explanation sometimes given 
is, that as the accommodation diminishes, the time at 
length arrives when the amount of accommodation 
at the patient's disposal is not sufficient to produce 
clear images ; he therefore relaxes his accommodation, 
and with it extreme convergence. 

Divergent Concomitant Strabismus exists when one 
eye only fixes the object looked at, the other deviat- 
ing outwards (Fig. 97) ; it is usually dependent on 
myopia, a state of refraction in which the converg- 
ence has to be used in excess of the accommodation if 
an image is to be formed on the macula of each eye ; 
but divergent strabismus may occur in any eye in 



216 



THE EFFRACTION OF THE EYE 



which binocular vision does not exist, as in some 
cases of high hyperinetropia or astigmatism ; or it 
may result from a too free division of the internal 
rectus muscle, in attempting to cure a case of con- 
vergent strabismus. Divergent strabismus is also 
occasionally met with in emmetropia and hyperine- 
tropia, and is then due to congenital insufficiency of 
the convergence. 

Fig. 97. 




Divergent concomitant strabismus is much less 
common than the convergent variety. 

In myopia the antero-posterior diameter of the 



DIVERGENT STRABISMUS 217 

eyeball is elongated, the range of movement is dimin- 
ished, and the extreme convergence which is neces- 
sary to enable the patient to see objects within his 
far point tires out the internal recti muscles, giving 
rise to muscular asthenopia : to relieve this one of 
the internal recti gives way, and the eye deviates 
outwards. 

Sometimes the deviation only takes place after the 
patient has been working some time and the eyes feel 
fatigued; in others it is only noticed when looking 
at objects beyond their far point. Soon, however, the 
squint becomes constant, and a divergent strabismus 
once established usually increases. 

In high myopia which is uncorrected by glasses, 
the patient has to hold objects so close to enable him 
to see them, that the necessary convergence becomes 
impossible, and binocular vision is therefore sacrificed. 

The treatment of concomitant squint may be divided 
into three parts — 

1. Optical. 

2. Operative. 

3. Educational. 

The optical treatment consists in prescribing glasses 
which correct any error of refraction, and prevent 
excessive accommodation. 

In concomitant convergent strabismus, when the 
squint has just commenced, and arises only under the 
influence of excessive accommodation necessary to 
enable the child to see near objects (periodic squint), 
then resting the eyes by allowing no near work to be 



218 THE REFRACTION OP THE EYE 

done, may suffice to remove the deviation, and so 
preserve binocular vision. 

It is obvious that such treatment cannot be indefi- 
nitely carried out ; therefore, it is essential that in 
all patients over three years of age the refractive 
condition of the eyes be accurately estimated under 
atropine, and the proper correcting glasses ordered 
for constant use, our object being to bring up the 
visual acuity of the patient to its highest standard. 

When the convergent strabismus has already 
become permanent we must keep the patient under 
atropine for a week or two, correcting his hyperme- 
tropia with glasses at the same time, while he must be 
cautioned to abstain as much as possible from looking 
at near objects, the mere impulse of convergence 
being sufficient to produce the squint. In some cases 
this treatment will cure the strabismus ; and if at the 
end of a year or so no recurrence of the squint has 
taken place, an attempt may be made to leave off the 
spectacles when out of doors, using them only for 
near work. 

If the case belong to the less common variety of 
squint — divergent with myopia — we endeavour to 
give the patient as near as possible his full correction 
for constant use. 

It will frequently be noticed that after the use of 
atropine the deviation may be diminished, or in 
slight cases it may have disappeared; this is due to 
accommodation being rendered impossible : these are 
the cases that are usually curable by glasses. 

Should the child be too young for spectacles (under 



STRABISMUS 219 

three years), we must endeavour to prevent the in- 
crease of the squint, and also prevent the deviating 
eye from becoming amblyopic ; this can best be done 
by keeping the child atropized for a few weeks at a 
time, and tying up the eye which does not squint for 
a short time each day, compelling the other to be 
used, and thus preserve its visual acuteness ; this has, 
of course, no effect on the deviation, the covered eye 
converging under the bandage. After the age of 
three, spectacles may be prescribed. 

Operative treatment. — In many cases, after the 
glasses have been worn for some months, the stra- 
bismus may still exist, and it may then be necessary 
to supplement the treatment by tenotomy. A free 
division of one muscle may cure a deviation of 15° : 
when a greater effect is required, both internal recti 
may be divided ; or an advancement of one of the 
muscles may be necessary, with or without a tenotomy 
of its antagonist. 

After the operation for convergent strabismus there 
should still remain a slight tendency to convergence 
when the glasses are removed ; because as the child 
approaches the age of maturity the excessive in- 
nervation of the internal recti may subside, and then 
there may be danger of a deviation of one eye out- 
wards. 

Educative treatment. — In most cases it will be 
found that even when the deviation of the eye has 
been corrected, binocular vision is not obtained, and 
the cure of the case cannot be considered complete so 
long as binocular vision is absent; great patience 



220 THE REFRACTION OP THE EYE 

and care will be required in carrying out the necessary 
exercises. 

Box stereoscopes are made for this purpose without 
prisms, but fitted with a clip at each sight-hole capable 
of taking the lenses of the ordinary trial box. The 
patient being emmetropic he will require in the clip 
a convex glass whose focal length corresponds with 
the length of the stereoscope; thus if it be 16 cm. 
long, he will require + 6 D. ; should the patient be 
hypermetropic, 3D.; then he will require + 9 D. ; 
if myopic 3 D., then + 3 D. would be the glass 
required : the object is to enable him to see the 
slide at the end of the stereoscope without accom- 
modating. 

A convenient slide may be made, composed of two 
vertical lines, one above and the other below the same 
horizontal line, so arranged that the two lines can be 
made to recede or approach each other : this test 
object is placed in the box instead of the ordinary 
views. The two lines being now separated to a dis- 
tance equal to that between the two eyes, and the 
clips containing the necessary convex glasses, the 
patient will see the lines without accommodation or 
convergence, and should succeed in fusing the two 
lines into one. When this is done binocular vision is 
obtained with parallelism of the lines of fixation. We 
endeavour at future trials to obtain fusion with an 
equal amount of convergence and accommodation. 
This is done by sliding the two lines towards each 
other about 1 cm. ; this will call into play something 
like 1 m. a. of convergence ; we then diminish the 



STRABISMUS 



221 



convex glass 1 D., so that the amount of accommoda- 
tion provoked (1 D.) may correspond to the amount of 

Fig. 98 (Landolt). 




In this way we slide the lines nearer 
and nearer together, diminishing the -f- glasses at the 
same time, until the two form one vertical line, then 
binocular vision is obtained with 6 m. a. of convergence 
and with 6 D. of accommodation ; when this point has 
been reached, stereoscopic pictures may be used as 
slides. 

When binocular vision does not exist, the patient 
should be taught to use the squinting eye, and this 
can be conveniently done by having a small, black, 
metal disc made to slip over the spectacle-glass of the 
fixing eye ; with this eye excluded, the patient should 
read with the defective eye for fifteen minutes twice a 
day, commencing with large print where a high degree 
of amblyopia is present, and decreasing the size of the 
print gradually as the eye improves. In very young 
children the disc may be worn over the fixiug eye for 
a part of each day, and the child encouraged to run 
about and play. 

Another most useful exercise is the reading bar, 
which may be used for a short time twice a day 



222 THE KEFRACTION OF THE EYE 

— a pencil, the finger, or a small strip of metal will 
answer the purpose ; this is merely to be held about 
6 or 7 cm. in front of the book when reading ; if only 
one eye is used the patient stops when he comes to 
that part of the line covered by the bar j when both 
eyes are in use, then he goes on reading without any 
interruption. 

Herring's drop test is perhaps the best test for 
finding out the presence of binocular vision. 

Paralytic strabismus does not come within the 
province of this work. 

See Case 22, page 255. 



ASTHENOPIA 223 



CHAPTEE XI 



ASTHENOPIA 



Asthenopia ('A, priv. ; aOtvog, strength ; &\p, the 
eye), or weak sight, is a term used to designate a 
group of symptoms caused by fatigue or strain of 
some part of the eye or its muscles. 

Asthenopia frequently accompanies hypermetropia, 
myopia, and astigmatism, and reference has often 
been made to it when speaking of these errors of 
refraction. It is also met with in cases where no 
ametropia exists, and may then be caused by over- 
fatigue or diminished power of the ciliary muscles, 
weakness of the internal recti, or exhaustion of an 
over- sensitive retina. 

Asthenopia shows itself by the inability to sustain 
long and continuous near work, and is accompanied 
with more or less pain ; the condition is a very common 
one, and it may be stated with confidence that pain 
in the eyes, unconnected with inflammation, is almost 
invariably due to asthenopia, and but seldom to any 
deep-seated disease. The more acute the pain, the 
more does it point to asthenopia ; as a rule, however, 
the pain is not very severe : it may be situated in the 
eyes themselves, or around the orbits, and is always 



22-4 THE P.EFBACTIOX OF THE EYE 

increased when the eves are used for near objects : in 
some cases no pain is felt, but after reading for a 
while the type becomes indistinct or double, so that 
the patient has : stop and look about the room, or 
rub his eyes, after which he may be able to resume 
reading for a short time, to be again quickly inter- 
rupted by a repetition of the same £ mptoms. If the 
work be still persisted in, the pain around the 
increases, there is photophobia and laehrvraarion, a 
sensation of dazzling and dimness, more or less con- 
junctival congestion, the eyes becoming red and 
irritable ; all these symptoms are liable to be worse in 
the evening after a day's work, when there is the 

additional disadvantage of an artificial lisrht. which 

_ 

may be hot and irritating. 

Headache is often a prominent symptom of asthe- 
nopia ; it may take the form of heaviness or pain 
over the brow (which may or may not be combined 
with general headache) ; it is often periodic in 
character, and is always made worse by reading; 
frequently there is a tender spot on the top of the 
head, or pain in the occipital region, occasionally also 
there is pain in the back of the neck. These symptoms 
may be associated with dyspepsia, palpitation, and 
vomiting, and in some cases with obstinate insomnia. 

This train of symptoms has occasionally been 
severe as to lead to the diagnosis of brain disease, 
hence it is a good rule to test the refraction under 
atropine in all cases of persistent headache not giving 
-rdiuary medical treatment, and it must be 
remembered that a very slight amount of astigmatism 



ACCOMMODATIVE ASTHENOPIA 225 

left uncorrected, even though the chief portion of it 
may be corrected, will be sufficient in some cases to 
keep up the headache. 

There is little doubt that frequently reflex nervous 
disorders are caused by asthenopia. 

Asthenopia may be divided into — 

1. Accommodative. 

2. Muscular. 

3. Retinal. 

Accommodative Asthenopia is exceedingly common, 
and is due to fatigue of the ciliary muscle ; and may 
be met with in emmetropia or ametropia. It makes 
itself known by an inability to maintain the necessary 
accommodation, and may arise (a) from a weak con- 
dition of the ciliary muscle, (b) from excessive use, 
as in hypermetropia, (c) from unequal demand, as in 
astigmatism, (d) from unequal demand in the two 
eyes, as in anisometropia, (e) from diminished elasticity 
of the lens, as in presbyopia. 

When Donders discovered the common occurrence 
of hypermetropia, he soon became aware of the inti- 
mate connection which existed between it and asthe- 
nopia, and was at first inclined to attribute every 
case to this cause. Where no manifest hyperme- 
tropia was present he gave a solution of atropine to 
paralyse the accommodation, feeling confident that 
some latent hypermetropia would then display itself ; 
such cases were usually completely cured by proper 
convex glasses. This accommodative asthenopia is 
due in great measure to the constant and excessive 
action of the ciliary muscle, but partly also to the 

15 



226 THE KK FRACTION OF THE EVE 

abnormal relations existing between the two func- 
tions, accommodation and convergence ; this state- 
ment is supported by the fact that hypernietropes 
who squint seldom suffer from asthenopia. An em- 
metrope looking at distant objects does so without 
any accommodation, the ciliary muscle being passive j 
but the hypernietrope has to use his ciliary muscle 
even for distant objects, and therefore much more 
for reading or near work ; so that the ciliary muscle 
practically gets no rest. A young and vigorous 
person whose hypermetropia is not very high may 
resist asthenopia for a long time, but as he gets 
older, or if his health suffer from any cause, symptoms 
of this disorder are apt to appear, and when once 
established they may continue, notwithstanding im- 
provement in the patient's general condition. In 
women asthenopia is very liable to come on during 
lactation. 

Treatment. — We order such glasses as are neces- 
sary to correct the refraction according to the rules 
given. In some cases where convex glasses do not 
produce the desired relief, prisms of 2 C bases inwards 
combined with the spherical correction are of great 
use, or in slight cases we place the convex glase - 
somewhat near together, so that the patient looks 
through the outer part of them (Fig. lOlj. This plan 
is frequently very useful in presbyopia. Here the 
asthenopia is due to a greater muscular effort being 
required to produce the necessary change in the 
shape of the less elastic lens, and perhaps, also, in 
part to the difficulty of maintaining an exact state of 



MUSCULAR ASTHENOPIA 227 

equilibrium between the internal and external recti 
muscles. 

In the hypermetrope there is a want of harmony 
between the accommodation and the convergence, 
the two functions having to be used in unequal 
degrees ; and when we correct his refraction with 
glasses he will have to use these two functions 
equally, or at least in different proportions from that 
to which he has been accustomed. Many people are 
able at once to adapt themselves to this new state of 
affairs; but there are others in whom the force of 
habit is so strong that they cannot thus throw off the 
acquired one of using the accommodation in excess of 
the convergence. You must not, therefore, be dis- 
couraged if occasionally your patient is not at once 
and completely relieved of his asthenopia, as soon 
as you have given him convex spectacles. A fort- 
night's trial should be made before we can decide 
that such spectacles will not relieve the patient of his 
asthenopia. 

Asthenopia depends much upon the nervous system 
of the individual; in some, a very slight amount 
of astigmatism will produce accommodative asthe- 
nopia ; one hypermetrope will have no uncomfortable 
feelings, while another, whose condition seems exactly 
similar, will suffer much, so that it is essential to attend 
to the patient's general health. 

Muscular Asthenopia is due to fatigue of some of 
the external muscles of the eyeballs. 

The two eyes should be in a condition of perfect 
muscular equilibrium in every position of the eyes ; 



228 THE REFRACT rON OK THE EYE 

when this is not the case, it may be detected by 
covering one eye with a screen, while the other is made 
to fix a small object, such as the point of a pen, held 
about 30 cm. away; the covered eye should be 
accurately adjusted for the object, although it no 
longer sees it : when this is not the case the covered 
eye may deviate inwards or outwards ; and if the 
screen now be withdrawn a movement of readjust- 
ment takes place. 

Maddox has pointed out that in practice it will 
be found that on using the two functions of accom- 
modation and convergence, the convergence has a 
tendency to lag behind the accommodation and re- 
quires the further stimulus of fusion to ensure the 
exact direction of the visual axes, so that when one 
eye is covered the other may deviate outwards a few 
degrees and still be considered within normal limits. 

When this deviation of the covered eye is greater 
than 5°, then there is disturbance of the muscular 
equilibrium, and the condition is spoken of as in- 
sufficiency or latent convergence or divergence. 

When insufficiency exists the results which follow 
vary. 

1 . Slight degrees may be corrected by increased 
innervation of the weak muscles, and so give rise to 
no symptoms. 

2. Muscular asthenopia may result from excessive 
innervation ; this may at once be relieved by closing 
one eye. 

3. Or it may pass on to concomitant squint. 
Heterophoria is the term now generally employed 



MUSCULAR ASTHENOPIA 229 

to express a disturbance of the equilibrium of the 
muscles of the eyeball, and may be divided into — 

1. Exophoria. 

2. Esophoria. 

3. Hyperphoria. 

4. Insufficiency of the oblique muscles. 

a. Hyperesophoria. 
0. Hyper exophoria. 

Exophoria, or latent divergence ; one eye tends to 
turn out, and is only prevented from doing so by 
increased innervation of the internal recti muscles, 
there is, in fact, insufficiency of these muscles, resulting 
in a strain of the convergence. 

Exophoria is the commonest of these defects, and 
is most frequently associated with myopia, though it 
occasionally occurs in emmetropia or even hyperme- 
tropia ; it is characterised by inability to maintain 
prolonged convergence. The patient complains that 
the eyes become tired, especially during the evenings, 
reading or writing cannot be continued for any 
length of time ; he has pain in and around the 
eyes, with headache ; objects look dim and indis- 
tinct, and there is a tendency to see things double ; 
sometimes the patient experiences a sensation as if 
one eye had turned outwards, which may really be 
the case; frequently the patient finds relief by closing- 
one eye. 

In myopia the disturbance of the two functions 
accommodation and convergence, may in some mea- 
sure tend to the production of this form of asthenopia. 
Thus a patient with 4 D. of myopia has his punctum 



230 THE REFRACTION OF THE EYE 

remotum at 25 cm. ; to see an object at that distance 
he must converge to that point, maintaining at the 
same time a passive condition of his accommodation. 

Strain of the internal recti muscles is essentially 
dependent upon binocular vision ; no convergent strain 
can exist where binocular vision is not present. 

Esophoria, or latent convergence, is due to insuffi- 
ciency of the external recti, and is seldom seen, for it 
quickly passes on to convergent squint with loss of 
binocular vision ; here, as a rule, we are dealing with 
excessive innervation of the internal recti muscles 
rather than with an insufficiency of their opponents. 

Hyperphoria, one eye tends to deviate upwards j 
this is usually associated with esophoria, but may 
exist alone ; this is not a very uncommon defect. 

Insufficiency of the obliques I have never seen, and 
no cases have been recorded in this country, though 
they seem to occur in America. 

To test and record the amount of latent deviation, 
the glass-rod test described on page 46, may be em- 
ployed. The patient is directed to look at the flame 
of a candle 6 metres away ; immediately behind the 
flame is a scale for measuring the amount of devia- 
tion; the glass rod is then placed horizontally before 
the right eye in testing for exophoria or esophoria : 
if the streak of light appear on the right of the 
candle homonymous diplopia is present, and the con- 
dition of the eyes is one of convergence : whereas if 
the streak I 3 ite position on the left side of the 
candle, crossed diplopia is present, and the eyes are 
divergent : the number on the scale corresponding 



MUSCULAK ASTHENOPIA 231 

with the position of the streak of light indicates the 
amount of convergence or divergence. When em- 
ployed for Iryperphoria the rod must be placed before 
the eye vertically. 

Another test for detecting insufficiency of the con- 
vergence is sometimes employed. 

Place a prism of about 15°, with its base down- 
wards, in a spectacle frame before one 
eye : by this means we cause a displace- 
ment of the eye upwards which produces 
vertical diplopia. ' The patient is now 
directed to look steadily at a card, on 
which is drawn a line with a dot in its 
centre, placed at the patient's ordinary 
reading distance (Fig. 99) . Naturally he 
will see two dots. If he see one line only 
with two dots on it, his muscles are 
assumed to be of the normal strength ; 
if, however, two lines are seen with a dot 
on each, then insufficiency exists, and 
the strength of the prism which is neces- 
sary with its base inwards to produce fusion 
is the measure of the insufficiency. 

The most satisfactory test for muscular 
insufficiency is, however, the rod test ; and 
having recorded the amount of latent con- 
vergence or divergence for distance, we next ascertain 
if there is any latent deviation in near vision. 

A prism of 12° base upwards being placed before 
the right eye in a spectacle frame, the scale is held 
i metre from the eyes. The scale used by Maddox 



232 THE REPBA.CTION OF THE EYE 

consists of a horizontal line in the centre of which is 
an arrow pointing upwards (Fig. 100). The line is 
divided into metre angles which are marked by figures, 
black on the right of the arrow, red on the left. The 
prism of course causes two arrows and two lines to be 
seen : the patient is directed to fix the fine print just 
below the arrow, and if there is no deviation the 
two arrows will be seen, one immediately below the 
other; if the lower one point to the right (black 
figures) there is latent convergence, but if to the 
left (red figures) latent divergence for this distance : 
the amount of deviation is read off the scale and duly 
recorded. 

Treatment. — In cases of myopia we give such 
glasses as correct the refraction to be worn con- 
stantly; these frequently succeed in relieving the 
asthenopia. When this is not the case, weak prisms 
bases inwards, by which we diminish the amount of 
convergence necessary, often give instant relief. It 
is sometimes useful to combine the prisms with 
concave glasses, or by separating the glasses some- 
what widely we may produce the same result. Fig. 
101 shows concave spectacles, which act as prisms 
by being slightly separated; convex glasses have 
the same effect when placed so close together that the 
patient looks through the outer part of the lenses. 
Or the lenses, instead of being thus displaced, may be 
decentred by so grinding the glass that its optical 
centre is not the centre of the glass. These de- 
centred glasses are spoken of as prism ospheres, and 
when ordered the amount of decentration should be 



*— I 



CO 



CaJ 



to 



^— J 



RETINAL ASTHENOPIA 233 

stated in millimetres on the order card ; the more the 
glasses are decentred the greater will be the pris- 
matic effect produced (page 194). 

When actual divergence of one eye takes place, 
advancement of the internal rectus, with or without 
division of its antagonist, may be necessary. 

Fig. 101. 




Retinal Asthenopia is due to fatigue of the retina. 

In addition to those cases of asthenopia occurring 
with hypermetropia, myopia, and astigmatism, which 
should be relieved by the proper optical correction 



234 THE REFRACTION OF THE EYE 

restoring the balance of the extra- and intra-ocular 
muscular systems, every one will occasionally meet 
with cases where there is intense discomfort and 
inability to read or do near work for any length of 
time, but where no ametropia exists, as proved by 
placing the patient under atropine and then testing 
the refraction. The visual acuteness is often very 
good, frequently rising to f or higher. The pain 
complained of is usually at the back of the eyes, with 
more or less headache, photophobia, lachrymation, a 
feeling of tension and heat, together with itching 
and pricking of the eyelids. Sometimes the chief 
symptom complained of is the conjunctival irritation 
accompanied with increased secretion. 

These cases are always exceedingly troublesome and 
difficult to cure ; they occur most frequently among 
young unmarried girls of an hysterical or nervous 
temperament. Less frequently men are affected, and 
then it is chiefly amongst those who are feeble, hypo- 
chondriacal, and nervous. 

With the ophthalmoscope the eyes may appear 
quite normal, or the retinal veins may be full with or 
without some slight haziness of the edges of the discs ; 
the perimeter may reveal a spiral contraction of the 
visual fields due to progressive exhaustion of the 
retina, which probably follows reflex contraction of 
the retinal vessels. 

Retinal asthenopia may be attributed to long hours 
of near work which has been done by artificial light, 
especially in those who have been previously reduced 
by some lowering illness. I have met with several 



RETINAL ASTHENOPIA 235 

cases amongst those making gold lace, and no doubt 
the bright materials here worked with had something 
to do with the production of the retinal hyperes- 
thesia. 

It seems generally accepted by all authorities on 
this subject that in most cases the nervous system is 
exceedingly sensitive and unstable. 

Sometimes the asthenopia is distinctly of reflex 
origin, produced by disturbance of the internal organs; 
when leucorrhoea exists in young unmarried women, 
with troublesome asthenopia, masturbation may be 
suspected. Irritation of the fifth nerve from carious 
teeth may also act as the exciting cause. 

Treatment. — Complete abstinence from near work 
does not always give relief, nor is this abstinence to be 
encouraged. A slight amount of regular work should 
be done every day, with rest for the eyes during the 
evening. Usually some form of nerve tonic is indicated, 
with plenty of outdoor exercise and the avoidance of 
strong light or places where there is a great glare, 
such as the sea-side in summer. Tinted glasses are 
to be avoided, since they merely tend to increase the 
hyperesthesia of the retina. 



236 THE REFRACTION OP THE EYK 



CHAPTER XII 



SPECTACLES 



Having referred to the subject of spectacles when 
considering* the correction of the different forms of 
ametropia, I will now briefly recapitulate what was 
then said, even at the risk of being accused of un- 
necessary repetition. 

Hyper m etropia. — So long as -| can be read with 
each eye, no glass is necessary for distant vision ; for 
reading and near work we give such glasses as 
correct the manifest and ^ of the latent hyperme- 
tropia. 

If distant vision be improved by convex glasses, 
then these may be prescribed. 

In hypermetropia complicated with strabismus we 
estimate the total hypermetropia under atropine, then 
give the full correction to be worn constantly. 

Myopia. — In cases of low degree we may prescribe 
folders for distance, and allow the patient to read and 
write without glasses if only he keep a sufficient dis- 
tance (30 cm.) from his book and suffer no inconveni- 
ence. In medium degrees the best results often 
ensue when the full correction is worn constantly 
both for near and distant objects. 



SPECTACLES 237 

Where the myopia is of high degree the full cor- 
rection may be satisfactory for distance, but uncom- 
fortable or impossible for reading, owing to the 
accommodation being insufficient. These glasses also 
have the disadvantage of diminishing the size of 
objects ; here we give two pairs of spectacles, one for 
distance, and a weaker pair for reading. 

Astigmatism-. — Our object is to give as near as 
possible the full correctiou (found by atropizing the 
patient) ; these glasses should be worn constantly. 

Atropine is seldom necessary in patients over twenty 
years of age. Homatropine and cocaine is usually 
sufficient in older people. 

Convex glasses render parallel rays which pass 
through them convergent ; they add therefore to the 
refraction of the dioptric system, and are called posi- 
tive. 

Concave glasses render parallel rays divergent ; 
they therefore diminish the refraction of the dioptric 
system, and are called negative. 

Convex glasses add to the quantity of light entering 
the eye, while concave glasses diminish it. 

The size of the image is modified : thus positive 
glasses bring forward the nodal point, and so increase 
the size of the image ; while negative glasses carry the 
nodal point backwards, and so diminish the size of the 
image. 

Glasses may be made of rock crystal (commonly 
called pebbles) or crown glass. Those made from the 
former material have the advantage of being harder, 
and are therefore less likely to be scratched than 



238 THE RKFKACTION OF THE EYE 

glass ; the weight is much the same in both cases. 
Pebbles absorb more heat, and unless cut exactly at 
right angles to their optic axis they are apt to refract 
unequally; besides, it is difficult to get rock crystal 
free from striae, so that lenses made from good crown 
glass are quite equal to the best pebbles, very much 
cheaper, and almost universally used. 

The method of mounting spectacle glasses is of the 
greatest importance ; they must be accurately centred 
in frames that are light, strong, and fit comfortably, 
otherwise the good effect of the most carefully chosen 
correction may be entirely frustrated by a faulty 
position of the glasses, or even a fresh source of eye- 
strain may be introduced. Glasses for constant use 
should be in the same plane and centred for distance ; 
those intended for near work only should converge 
slightly, and be centred for the reading distance. 

Each lens should be in the first focal plane of the 
eye, that is 13 7 mm. in front of the cornea. When this 
is the case, the images formed on the retina will be 
of the same size as in emmetropia. 

The bridge of the frame should be moulded to suit 
the shape of the nose, resting on it by a broadish 
surface so as not to cut or indent the skin, while the 
glasses should be a sufficient distance from the eyes to 
just clear the lashes. The sides of the frames should 
pass back immediately above the ears, and in many 
cases, especially where glasses are required to be worn 
constantly,, they may with advantage bend directly 
round the ears, fitting the posterior part of the 
concha; these ear-pieces may be made of twisted 



SPECTACLES 239 

wire, which gives them considerable elasticity and 
strength. The frames may be made of gold or steel ; 
the latter material has the disadvantage of rnsting 
easily. 

When glasses are worn for myopia or hyper- 
metropia they should not be further from the eye 
than 13' 7 mm. For presbyopia the person may be 
allowed to suit his own convenience and comfort, 
2 1 cm. being an ordinary distance. 

Single glasses may occasionally be allowed in low 
degrees of myopia for looking at distant objects. 
They have the disadvantage of encouraging mon- 
ocular vision, and sometimes one eye is used so 
entirely that the sight in the other may deteriorate 
from want of sufficient use. 

Folders (pince-nez), of which there are many varie- 
ties, may be used in some cases of hypermetropia 
and myopia ; many presbyopes find them very con- 
venient for reading. Spectacles are as a rule to be 
preferred, especially in children, since they are more 
accurately centred and fit better. 

In cases of astigmatism it has hitherto been the 
custom to order spectacles and not folders, as in the 
latter it is difficult to be certain that the cylindrical 
glasses are always in their proper axis; but lately 
several ingenious pince-nez have been brought out 
which are free from this objection. That form in 
which the glasses slide on a horizontal bar are so 
arranged that they fit on the nose very easily, and 
are extremely comfortable, and may be recom- 
mended in many cases. 



240 THE BEFKACTION OF TFTE ETK 

In addition to concave, convex, and cylindrical 
glasses, others are sometimes used. 

Stenopaic spectacles consist of an opaque screen 
with a small central aperture which may be of any 
shape to suit a particular case, so that all the peri- 
pheral rays are cut off, only those that are in the 
visual axis being allowed to pass through. They can 
be combined with convex or concave glasses, and are 
sometimes exceedingly useful in cases of leucomata, 
nebulae, irregular astigmatism, conical cornea, etc., 
where the vision is much disturbed. 

Prismatic spectacles may consist of prisms alone, or 
they may be in combination with concave or convex 
lenses. It is not convenient to use prisms much 
stronger than 3° or 4°, owing to their weight. They 
are useful in certain cases of paralysis of muscles, to 
correct the diplopia, and in some cases of hyper- 
metropia, myopia, and astigmatism which are not 
relieved by their proper correction; prisms are also 
used for testing the ocular muscles and for detecting 
malingerers. When ordered in cases of asthenopia 
with errors of refraction, they may be combined with 
the glasses which correct these errors (p. 194). 

Pantoscopic Glasses. — The upper half of these 
glasses have one focus, the lower half another. Thus 
a presbyopic person may also be myopic. The upper 
half of the glass would then be concave for distance, 
the lower half convex for near work. Painters some- 
times find such glasses very useful. 

Tinted glasses are often required for diminishing 
excessive light, especially where there is irritation 



SPECTACLES 2 41 

or inflammation of the retina ; they are also useful 
in some cases of photophobia, arising from various 
causes, as myopia, etc. Where the aim is to relieve 
the retina without injuring the distinctness of vision 
the light blue glasses are the best, as they cut off 
the orange rays; where the object is to act on the 
quantity and not the quality of the light, smoke- 
coloured glasses are to be preferred. Tinted glasses 
sometimes do real harm, as in cases of asthenopia, by 
increasing the sensitiveness of the retina ; they are 
always somewhat heating to the eyes, in proportion 
to the amount of rays they absorb. We sometimes 
combine them with convex or concave glasses. 

There are also various forms of 'protectors ; those 
hollowed out like a watch-glass, so as to fit closely, 
are to be preferred to those with wire sides called 
goggles, or those with sides of glass, which have the 
disadvantage of being too heavy. Workmen wear 
different sorts of protectors to keep off dust, frag- 
ments of stone, etc., which may be made of glass, talc, 
or other material. 

Shot-proof spectacles are also made for sports- 
men. 

It is sometimes necessary to find out and record 
the strength of glasses that are being worn; this is 
easily done. If convex, we take a concave glass out 
of the trial box, hold it against the glass we are 
trying, and look through them at a line, e. g. the bar 
of a window or any similar object. We move the 
glasses to and fro in front of the eye : if the line 
remain immoveable the neutralisation is complete; 

16 



242 THE KEFRACTION OF THE EYE 

if it move in the same direction the concave glass is 
too strong; if in the opposite direction it is not 
strong enough. 

A small lens measure is now made for estimatino- 

o 

the strength of lenses. 



Cases 

Commence the examination in a systematic manner. 

First, notice the general appearance of the patient, 
then the shape of the head and face. Next the 
eyes, as to whether they are large and prominent, or 
small and sunken-looking. Listen patiently to the 
sufferer's complaints, and having submitted to this 
ordeal, test the acuteness of vision of each eye 
separately, and afterwards together, writing down 
the result, remembering always to commence with 
convex glasses. Then place the near type in the 
patient's hand, noting the number of the type and 
the distance at which it can be read. Next pass on 
to the ophthalmoscope, first applying the " retino- 
scopic test," then the " indirect examination," and 
finally the " direct method," first at a distance, and 
then close to the eye. If any ametropia exist, the 
advisability of paralysing the accommodation with 
atropine must be considered. 

In order to illustrate this method of examination, I 
will give a few cases in addition to those which will 
be found at the end of the chapter on Retinoscopy. 

Case 10. Hypermetropia. — E. M — , a young woman, 



cases 243 

a book-folder, set. 17, suffering from tinea tarsi, 
complains that her eyes get very tired at nig'ht, so 
much so, in fact, that she is unable to read for any 
length of time. Her general appearance is healthy, 
and nothing special is noticed about her face, except 
that the eyes are small. The acuteness of vision for 
both eyes is normal. On placing + 1 D. in front of 
the right eye, -| is seen more distinctly than without, 
with + 2 D. -| is still read, with + 2'5 D. vision is 
not so good ; the same result is obtained with the 
left eye. + 2D. for each eye is the strongest convex 
glass with which -f can be read, and is therefore the 
measure of her Hm. ; on trying the two eyes together 
-|- 2*5 D. still gives -|. We record it thus : 

R.V.fH,,2D.=n fim2 , Di 
L.V. f Hm. 2 D.=f J b 

On placing the patient in the dark room, and 
directing her to look at some distant object or at a 
black wall, so as to relax as much as possible the 
accommodation, with the concave mirror the shadow 
we perceive moves slowly against the mirror. We put 
+ 2 D. in a spectacle frame, in front of the eye; the 
shadow is more distinct, and moves more quickly. 
We try stronger glasses, and then find that -f 3*5 D. 
is the highest with which we still get a reverse 
shadow. Both eyes are alike. 

Next examine with the ophthalmoscope. By the 
indirect method the disc becomes smaller on with- 
drawing the objective from the eye. With the 
mirror alone at a distance, we see an image of the 



244 THE REFRACTION OF THE EYE 

disc which moves with the observer's head, proving 
the image to be an erect one. On approaching the 
eye the disc is not seen well, unless we pnt in force 
onr own accommodation. With onr accommodation 
suspended, we turn the wheel of the ophthalmoscope 
so as to bring forward convex glasses ; the clearness 
of the fundus is improved; + 4 D. is the strongest 
convex glass with which the details can be distinctly 
and clearly seen by myself. 

We might be satisfied with this result, assuming 
4 D. to be the amount of total hypermetropia, but in 
vouug people it is much more satisfactory to be able 
to record once and for all the total hypermetropia 
beyond doubt. Atropine (grs. iv to jj) was therefore 
ordered, one or two drops to be placed in both eyes 
three times a day for four days, warning her that 
she will be unable to see well, and that the pupils 
will be dilated during their use. We also recom- 
mend a shade to be worn to protect the eyes from the 

light. 

On her return she reads only -^ with each eye, 

and she now requires + 5 D. to enable her to read 

We also find with retinoscopy that + 5 D. is 

the strongest glass with which we get an opposite 

shad 

Our patient, therefore, has a total hypermetropia 
of 5 D., two dioptres of which were manifest, and 
three latent. For work and reading we order her 
spectacle- - 3 D. At present she will not require 
them for distance. About thirty she will probably 
require 1 Qcreased to + 4 D. ; about forty 



cases 245 

she may be able to bear her full correction, and may 
then begin to wear them constantly. 

We must remember that when atropine has been 
used it is necessary to take off 1 D. from the measure- 
ments thus found. 

Case 11. Myopia. — A young man, aet. 20, next 
presents himself. He has a long intellectual face 
with prominent nose; the palpebral apertures are 
wide ; and on directing him to look inwards as much 
as possible, the eyeballs seem elongated in the antero- 
posterior diameter. 

His eyes, he says, are excellent, but he is unable to 
recognise people as well as formerly. We test the 
acuteness of vision, and find that he reads g- 6 ^ with 
each eye. Convex glasses make even that line in- 
distinct. Our patient is probably myopic. We place 
in his hand the near type, and he reads No. 1 at 
once and easily. The farthest point at which he can 
read it is 25 cm. (W = 4 D.) ; - 4 D. should be the 
measure of his myopia. We try — 4 D., directing 
him again to look at the distant type. He reads with 
each eye -|; we reduce the glass to find the weakest 
with which he reads the same, and with — 3'5 D. he 
reads it, though hardly so well ; with — 3 D. he reads 
only -| ; — 3*5 D. is therefore the measure of his 
myopia, and we record it thus : 

R.V_6__ 3 .5 D . = e. 
L.V.3%-3-5D. = f. 

If we employ retinoscopy — 3'5 D. is the weakest 
concave glass with which a reverse shadow is pro- 
duced. 



246 THE REFRACTION OF THE EYE 

We next subject the ere to the indirect ophthalmo- 
scopic examination. The image of the disc becomes 
larger on placing the objective near the eye and 
gradually withdrawing it, and in addition we see also 
a slight myopic crescent on the apparent inner side 
of the disc. From this case, disc Xo. 1 was drawn 
(p. 147). 

With the mirror alone at a distance from the eye 
the disc cannot be well seen, because in our case the 
aerial image will be formed about 25 cm. in front of 
the patient's eye. To enable us to see this aerial 
image it is necessary we should be some 30 cm. away 
from it; so that we should require to be 25 -f 30 
= 55 cm. from the observed eye, and at that distance 
the illumination will be very weak. 

With the direct method the details appear blurred 
until we put on a concave glass by turning the wheel 
of our refracting ophthalmoscope. The weakest con- 
cave glass with which we are able to see the details 
of the fundus clearly is the measure of the myopia. 
Thus we have four distinct plans of measuring our 
case of myopia : 

1st. The farthest distance at which the near type is 
read, 25 cm. [Iff = 4 D.). 

2nd. The weakest concave glass which gives the 
greatest acuteness of vision'. 

3rd. The weakest concave glass with which we get 
a retinoscopic shadow moving in the opposite direc- 
tion to the movement of the concave mirror. 

4th. The weakest concave glass with which the 



cases 247 

details of the fundus can be distinctly seen by the 
direct method. 

Should any of these results vary much, we should 
suspect that the myopia is increased by spasm of the 
accommodation, and we atropize the patient in the 
manner before described, and at the end of four days 
go over the ground again, remembering that when 
atropine has been used, it is necessary to add on 
about — *5 D. to the glass found, because the ciliary 
muscle is probably never so completely relaxed as 
when it is under the influence of atropine. 

Having found, then, that our patient's myopia 
amounts to — 3*5 D., we give spectacles of that focus 
for constant use. In addition to ordering spectacles, 
we give him also some very important general direc- 
tions : he must always hold his book or work 35 cm. 
away, bring the work to his eyes, and not his eyes to 
the work ; writing should be done at a sloping desk, 
he should sit with his back to the window, so that 
the light comes over his left shoulder on to his work, 
and do as little near work as possible by artificial 
light. 

Case 12. Hype?- metropia and Presbyopia. — A gentle- 
man, get. 56, comes with the complaint that he cannot 
see to read as comfortably as formerly, though he 
sees distant objects well. We try his acuteness of 
vision, and find that he reads -J badly. With + 1 D. 
he sees much better, reading some of the letters of -|. 
We then try + 1*5 D., and these he rejects. Hence 
we conclude that he has Hm. 1 D. We know from his 
age that he will also be presbyopic 3 D., and we add 



248 THE REFRACTION OF THE EYE 

on to this + 1 D. for his hypermetropia, directing 
him to read the newspaper with + 4 D. for half an 
hour. He thinks these rather strong for him, as they 
make his eyes ache. With + 3*5 D. he feels quite 
comfortable, and we therefore give him + 3*5 D., 
telling him that he may require them changed for 
slightly stronger ones in about five years. 

Case 13. Paralysis of the Accommodation. — Kate 
L — , aet. 12, has been very ill from diphtheria, but is 
now much better. She complains that she is unable 
to read or work, though able to see distant objects 
well. The pupils are very large, and a,ct badly to 
light. Hence we suspect paralysis of the accommo- 
dation. We test her acuteness of vision, and she 
sees -§- with each eye. We try convex glasses *5 D., 
and she still reads |-, but 1 D. she rejects. Our dia- 
gnosis is therefore confirmed. We next find the 
weakest glass with which she is able to read, weakest 
because we are anxious to encourage the ciliary 
muscle to act, since by replacing it entirely we should 
prolong the patient's recovery. 

The glasses must be changed for weaker ones as 
the ciliary muscle recovers tone. 

We saw that she had a slight amount of hyperme- 
tropia, and also that there was some accommodation 
left, enough at least to correct this, otherwise she 
could not have read J without + "5. A tonic con- 
taining iron and strychnine was also prescribed. 

Case 14. Anisometropia. — A young woman, aet. 20, 
has never seen well, either at a distance or near at 
hand ; has tried spectacles of all sorts, but never been 



cases 249 

able to find any that suited her. The eyes look 
somewhat irritable, but there is nothing conspicuous 
about their size or shape. There is some want of 
symmetry about the face, the nose being deviated 
from the median line slightly to the left. 

We first try the acuteness of vision of the right eye. 
She reads -£%> an( l with. + 1 D. vision is somewhat 
improved; with + 1*5 D. it is made worse. Still 
armed with + ID. we direct the patient to look at 
the fan of radiating lines (Fig. 85). She sees plainly 
the horizontal lines, whilst all the others are more or 
less indistinct, the vertical line most so ; still looking 
at the horizontal line, we alternately hold in front of 
+ 1 D., which is before the eye under examination, 
-f- '25 D., which makes it worse, then — *25 D., which 
she says at once makes it perfectly clear and distinct. 
We therefore put down -f *75 as the correction for 
the vertical meridian, and pass on to the horizontal. 
Our patient is directed to look steadily at the vertical 
line. We try convex glasses, these improve it, + 3D. 
making it quite clear; a stronger glass than this 
renders it slightly indistinct. It is evident, there- 
fore, that her horizontal meridian is hypermetropic 
+ 3 D. We put up the correction found, + '75 D. 
sp., + 2*25 D. cylinder axis vertical, and direct her 
again to look at the distant type ; J is read, though 
with some difficulty. This result is not, however, 
reliable, and we proceed to confirm it by retinoscopy, 
obtaining + 2 D. for the vertical, and + 4 D. for the 
horizontal meridians. On trying this correction, 
however, the vision is not so good. We now test the 



250 THE REFRACTION OP THE EYE 

acuteness of vision in the left eye ; she sees -fg, and 
neither convex nor concave glasses improve it. On 
looking at the fan of radiating lines, all seem indis- 
tinct, and having thns far no data to go upon, we, 
instead of wasting time, at once pass on to retino- 
scopy. We get oblique shadows, the horizontal 
moving with the mirror, and the vertical against it ; 
here, then, is a case of mixed astigmatism. We find 
out that — 2 D. is the weakest concave glass with 
which we get a reverse shadow horizontally, and 
+ 3 D. the strongest convex lens with which an 
opposite shadow is still obtained in the vertical meri- 
dian, the degree of obliquity being about 25°. This 
result is noted down thus : 



3 D. 



/ - 2D. 

We therefore place in a spectacle frame + 3 D. 
spherical, combined with — 5 D. cylind., axis devi- 
ating outwards from the vertical 25°. With this cor- 
rection the patient at once reads y^-. We are not to 
be satisfied with this result, but give the patient a 
solution of sulphate of atropine, grs. iv to $], with 
directions to come again in four days. At the end of 
that time she returns, and we find with retinoscopy — 
+ 2-5 I). + 35 D. 

+ 4-5D. 

-2 D. 



CASES 251 

The right eye with this correction reads f readily, 
and the left also f, but rather slowly. This result is 
very satisfactory. We now allow the patient to 
recover from atropine, and at the end of a week 
confirm the result before ordering spectacles. Then 
for the right eye the best vision was obtained with 
+ 1*5 sp. o +2D. cy. axis vertical (-J) ; and for the 
left + 3D, spherical o - 5 D. cylind. axis 20° from 
the vertical (-§-)■ These spectacles were therefore 
ordered, and the patient directed to wear them con- 
stantly. 

Case 15. Anisometropia. — Jane W — , set. 30, pre- 
sents herself, complaining that the sight in her left 
eye has been gradually getting dim for some months. 
She is a small, healthy-looking woman, with nothing 
characteristic in her appearance. We test the acute- 
ness of vision : 

Right f Hm. 1 D. = f 

Left T 6 g-, not improved with spherical glasses. 

We try retinoscopy, but the pupils are so small 
that the result is not very satisfactory. We are, 
however, able to make out in the left eye a reverse 
shadow in the horizontal meridian, which -f 2 D. 
over-corrects, + 1'5D. being the highest glass with 
which we get an opposite shadow ; the vertical meri- 
dian appears emmetropic. There is, therefore, no 
doubt that the defective vision in this eye is due to 
astigmatism. The patient complains that the exami- 
nation has made her eyes ache, so we do not proceed 
further, but order a solution of hydrobromate of homa- 
tropine (2 grs. to the 3j) to be used every three hours, 



252 THE REFRACTION OF THE EYE 

and direct her to come again on the following day. 
Then the result with retinoscopy is — 

R, + 1-5 D. 

+ •5 D. 
L. + 2 I). 

We try this at the test type. 
R.J^+1-5D.=£. 

+ -5 D. sp. 
36 + 1*5 D. ry. axis vert 8 " 

We make a slight deduction from the sphere in 
each case for the homatropine, and order for constant 
use — 

R. + lD. sph. 

L. + 1*5 D. ry. axis vert. 

Case 16. Presbyopia. — John Gr — , aet. 50, has 
always enjoyed good sight ; he still sees distant 
objects well, but finds some difficulty in reading, 
especially during the evenings. 
R.V. $, no Hm. 

L.V. &, no Hm. 

We try him with + 2 D. for reading, and with 
these he sees perfectly ; this, therefore, is a simple 
case of presbyopia, requiring a pair of folders + 2 D. 
for reading,, writing, etc. 

Case 17. Hypermetropia and Presbyopia. — Mr. K — , 
net. 60, sees badly both near and distant objects ; 
he wears + 4 D. for reading, but they are not com- 
fortable. 

H.v. T « o -Hm.3I). = 5. 
L.V.&Hm.3D.=f 



cases 253 

He therefore wants + 3D. for distance ; and to 
find the glass he will require for reading, it is neces- 
sary to add on to this distance lens the glass he 
would require for presbyopia if he were an emme- 
trope, viz. + 4 D. We therefore try him with + 7 D., 
but these make his eyes ache ; we next try + 6*5 D., 
and with these he sees comfortably. 

This patient, then, requires two pairs of spec- 
tacles, — 

+ 3D. for distance ; 

+ 65 D. for reading, &c. 

Case 18. Myopia and Presbi/opia. — Mrs. C — , 83t. 

55, complains that her eyes become tired at night ; 

she has tried several pairs of spectacles, but without 

finding any that exactly suit her. 

R.V.J r .2D.=f 
L.V. 3 R ¥ -2D.=f. 

Our patient requires, therefore, this correction for 
distance, but she also wants spectacles for reading 
and near work ; an emmetrope of fifty-five requires 
presbyopic glasses + 3D.; she is, however, a myope 
of 2 D., so we have to deduct this from the presbyopic 
glass ( ( + 3 D.) + (- 2 D.) = + 1 D.), and try the 
+ 1 D. for reading. With these she is able to read 
the smallest type comfortably ; we therefore pre- 
scribe two pairs of spectacles, — 

— 2D. for distance j 
+ 1 D. for reading. 

Case 19. Myopia. — Annie C — , aet. 9, was brought 
because she was unable to see the black-board at 
school. 



254 THE REFKACT10N OF THE EYE 

L.V.i-2-5D.=|. 

After using atropine — 

R.V.3<V-3D. = f. 

Ordered spectacles for distance R. — 3 D., L. — 2 
D., with directions to present herself again in six 
months, when, should the myopia have increased, or 
should the child complain of asthenopia, it may be 
necessary to prescribe spectacles for constant use. 

Case 20. Simple Myopic Astigmatism. — Thomas 
J — , ast. 20, sees rather badly both near and distant 
objects. 

R.V. T \, not improved with spheres; with pin-hole- f. 
L.V. T *V, not improved with spheres ; with pin-hole = £. 

After atropine had been used for four days retino- 
scopy gave — 

, + lD. i + l D. 

R. 1 Em. L Em. 



R. + 1 D. cy. axis horiz. =£. 
L. + 1 D. cy. axis horiz. =£. 

After the atropine has passed off — 

R. — ID. cy. axis vert. =f. 
L.-1D. cy. axis vert. = |. 

This correction was given for constant use. 

Case 21. Compound Myopic Astigmatism. — Miss 
M — , aet. 13, has seemed short-sighted for the last 
year or two. Mother and father both have good 
sight. 



cases 255 

The pupils are large, so that retinoscopy can be 
easily carried out. 

-10 D. 
--7D. 





-10 D. 




R.— 


6D. 


L.— 


Ry --6D.sp. 

-4D. cy. axis horiz. 


= T 6 g-and 2 


-3 D. 


sp. 

cy. axis horiz. 


_ e 

12* 



On examination of the eyes with the ophthal- 
moscope the choroid is found to be exceedingly thin, 
there is a large crescent in both eyes, and in the right 
are three or four patches of choroiditis, with one 
haBmorrhage near the macula. 

The patient was ordered the full correction for 
distance, and advised to do no reading, writing, or 
near work for six months, then to return for inspec- 
tion ; she was also recommended to spend as much 
of her time as possible in the open air, and a mixture 
containing syrup of the iodide of iron was pre- 
scribed. 

Case 22. Concomitant Squint. — George W — , set. 5, 
has squinted inwards for the last three months. On 
covering the non-squinting eye and directing the 
little boy to look at the finger held a short distance 
from him, the deviating eye immediately righted 
itself and fixed the finger, the covered eye at the 
same time turning in. We prescribed a solution of 
sulphate of atropine to be applied to both eyes, and 
at the end of a week the patient is brought back : the 
squiut is now much less apparent, and with retino- 



256 THE REFRACTION OF THE EYE 

scopy we find 3*5 D. of hypermetropia in each eye. 
The direct examination gives the same result. We 
order our patient spectacles + 2*5 D. to be worn con- 
stantly. 

Case 23. Aphakia. — Thomas B — , aet. 50, game- 
keeper. Had the right lens removed for cataract 
nine months ago, and last week the opaque capsule 
remaining was needled. 

R.V. c + 11 D. = «, and with + 14 D. No. 1 of 
the near type was read with comfort ; the patient 
was therefore ordered the following spectacles : 

+ 11 D. for distance ; 
+ 14 D. for near work. 

These were arranged in a reversible frame, so that 
either glass could be brought in front of the right eye 
as occasion required. 



APPENDIX 257 



APPENDIX 

In the metrical system the unit of length is a 
metre, equal to 100 centimetres, 1000 millimetres, or 
40 English inches ; so that 1 inch is equal to 2-|- cen- 
timetres. A lens of 1 metre focus is called a dioptre, 
a lens of ^ a metre (50 cm.) is 2 D., -^ of a metre 
(10 cm.), 10 D., etc. 

In the old system the lenses were numbered 
according to their focal length in inches, a lens of 
1-inch focus being the unit; a lens of 2-inch focus 
was expressed by the fraction -J, one of 10-inch focus 
■^qj and so on. If we wish to convert a dioptric 
measurement into the corresponding inch measure- 
ment of the old system, we have only to remember 
that the unit 1 metre = 40 English inches, so that a 
glass of 1 D. = Jq in the old system, 2D. = ^= -^oj 
5 D. = ^- = i and so on. 

The table on the next page gives approximately 
the equivalent of each dioptre or part of a dioptre in 
English and French inches, and their focal length in 
centimetres. 



17 



258 



THE KEFRACTION OF THE EYE 



Dioptres. 


English inches. 


French inches. 


Centimetres. 


•25 


160 


146 


400 


•50 


80 


73 


200 


•75 


52 


50 


130 


r 


40 


36 


100 


125 


31 


29 


77 


150 


26 


24 


65 


175 


22 


21 


55 


2- 


20 


18 


50 


2-25 


17 


16 


43 


2-50 


16 


15 


40 


2-75 


14 


13 


35 


3- 


13 


12 


33 


3-50 


11 


10 


27 


4- 


10 


9 


25 


4-50 


9 


8 


22 


5- 


8 


7 


20 


5-50 


7 


6* 


17 


6- 


6| 


6 


16 


7' 


6 


5 


15 


8- 


5 


4* 


12J 


9- 


4* 


4 


11 


io- 


4 


3.^ 


10 


ii- 


H 


3* 


9 


12- 


Si 


3 


8 


13- 


3 


2f 


7i 


14- 


21 


2i 


7 


15- 


2i 


2£ 


6h 


16- 


2i 


2£ 


6 


18- 


2| 


2 


5* 


20- 


2 


-■-4 


5 



APPENDIX 259 

Regulations for Candidates for Commissions in the 
Army 

A candidate must be able to read at least -f^ with 
each eye separately without glasses, and this mast 
be capable of correction with glasses up to -J iu one 
eye and -^ in the other; he must also be able to read 
No. 1 of the near type with each eye without the aid 
of glasses. 

Squint, colour-blindness, or any serious disease of 
the eye renders the candidate ineligible. 

Navy 

A candidate must be able to read |- with each eye, 
and the near type at the distance for which it is 
marked, without glasses. 

Colour-blindness, squint, or any disease of the eye 
disqualifies. 

Indian Civil Service 

A candidate must be able to read -§- with one eye 
and -| with the other, with or without correcting 
lenses. 

Any disease of the fundus renders the candidate 
ineligible. Myopia, however, with a posterior sta- 
phyloma, may be passed if the ametropia do not 
exceed 2*5 D., and the candidate has a visual acute- 
ness equal to that stated above. 

Indian Medical Service 
The candidate must have a visual acuteness of -J in 
one eye and -^ in the other. Hypermetropia and 



260 THE KE FRACTION OP THE EYE 

myopia must not exceed 5 D., and then with the 
proper correction must come up to the above standard. 
Astigmatism does not disqualify a candidate, pro- 
vided the combined spherical and cylindrical glass 
does not exceed 5 D., and the visual acuteness equals 
■f- in one eye and -f^ in the other. Colour-blindness, 
ocular paralysis, or any active disease of the fundus, 
renders the candidate ineligible. 

Public Works 

Candidates for the Departments of Public Works, 
Survey, Forest, Telegraph, Railways, Factories, and 
Police of India must pass the following eyesight tests. 
If myopic, the defect must not exceed 2'5 D., and 
with this glass the candidate may read -| in one eye 
and -| in the other. If myopic astigmatism is present, 
the vision must reach the above standard with correct- 
ing glasses, and the combined spherical and cylindrical 
glass must not exceed 2*5 D. 

In hypermetropia and hypermetropic astigmatism 
an error of 4 D. is permissible provided that with this 
glass -| is read with one eye, and -§ with the other. 

Any disease of the eye may disqualify. 

English Railways 

There is, unfortunately, no uniform standard for 
our railways ; each company has its own standard, in 
many cases a very low one : every engine driver 
should have at least — - in each eye without glasses, 
and normal colour vision. 



TEST TYPES 261 



TEST TYPES 



No. 1. 25cm. 



ion. of U» logild 



that Aot because we thought 
ie n sufficiently secured. Yet 



No. 2. 33cm. 



on itself to be the supreme criminal judicature in political cases." Warm eulogies were pronounced on the 
ancient national mode of trial by twelve good men and true ; and indeed the advantages of that mode of trial in 
political cases are obvious. The prisoner is allowed to challenge any number of jurors with cause, and a 
considerable number without, cause. The twelve, from the moment at which they are invested with their short 
magistracy, till the moment when they lay it down, are kept separate from the rest of the community. Every 
precaution is taken to prevent any agent of power from soliciting or corrupting them. Every one of them 



No. 3. 50cm. 

must hear every word of Ihe evidence and every argument used on either side. The case is then summed up 
by a judge who knows that, if he is guilty of partiality, he may be called to account by the great inquest of 
the nation. In the trial of Fenwick at the bar of the IJouse of Commons all these securities were wanting. 
Some hundreds, of gentlemen, every one of whom had much more than half made up his mind before the cass 
was opened, performed the functions both of judge and jury. They were not restrained, as a judge is restrained, 



* The number indicates the distance at which the type should be seen 
by a normal eye. 



262 TEST TYPES 



No. 4. 75cm. 



by the sense of responsibility ; for who was to punish a Parliament ? They were 
not selected, as a jury is selected, in a manner which enables the culprit to exclude 
his personal and political enemies. The arbiters of his fate came in and went out 
as they chose They heard a fragment here aiul there of what was said against him, 



No. 5. lm. 

and a fragment here and there of what was said in his favour. During 
the progress of the bill they were exposed to every species of influence. 
One member was threatened by the electors of his borough with the 
loss of his seat: another might obtain a frigate for his brother from 



No. 6. l-25m. 

Russell: the vote of a third might be secured by the 
caresses and burgundy of Wharton. In the debates arts 
were practised and passions excited which are unknown 
to well -constituted tribunals, but from which no great 



No. 7. l'5m. 

popular assembly divided into parties ever was or 
ever will be free. The rhetoric of one orator called 



TEST TYPES 263 

No. 8. 2m. 

forth loud cries of " Hear him." Another 
was coughed and scraped down. A third 

No. 9. 2'5m. 

spoke against time in order 
that his friends who were 

No. 10. 3-5m. 

supping might come 

No. 11. 5m. 

in to divide. If 

No. 12. 7m. 

prominent 



264 



TEST TYPES 





ZURRY &, PA 



London/, Liverpool . 



N E 

D -24 

D E Z 

D = 18 

N E V T 



L O E Z 



F A E Z L V 



Z F E V O T P 



D-5 



LTOADFZLV 



A 



INDEX 



Abducting prisms, 45, 240 
Accommodation, 82, 187 

absolute, 40, 213 

amplitude of, 37 

at different ages, 41 

binocular, 40 

diminution of, 40, 167 

of emmetropes, 87 

of hypermetropes, 38 

of myopes, 39 

paralysis of, 195 

produced by, 33 

range of, 37 

relative, 40, 50 

spasm of, 197 
Accommodative asthenopia, 124, 132, 225 
Acquired hypermetropia, 128 
Acuteness of vision, 29, 55 

in hypermetropia, 126 

in myopia, 14b 

in astigmatism, 165 

diminishes with age, 187 
Ad ducting prism, 46, 240 
Aerial image, 73, 246 
Alternating strabismus, 206 
Amblyopia, 211 
Ametropia, 26 
Amplitude of accommodation, 37 

of convergence, 4i 
Anderson, Dr Tempest, 182 
Angle 0,41,123,145, 2ul 

y, 203 

metrical, of convergence, 44 

of deviation, 9 

of strabismus, 207 

principal, 9 

visual, 29, 56 
Anisometropia, 29, 184 

correction of, 185 

treatment of, 185 
Anterior focal point, 24 

focus, 10 
Aphakia, 132 

case of, 256 

test for, 132 

treatment of, 133 
Apparent strabismus, 123, 200 
Appendix, 257 



Army, regulations for, 259 
Asthenopia, 124, 23 

accommodative, 124, 132, 225 

muscular, 146, 227 

retinal, 233 

retinal veins in, 234 
Astigmatism, 29, 156 

causes of, 164 

compound hypermetropic, 161 

compound myopic, 161 

estimation of, 167 

irregular, 157 

mixed, 161 

principal meridians in, 158, 168 

regular, 156 

shape ot disc in, 170 

simple hypermetropic, 161 

simple myopic, 161 

symptom's of, 164 

treatment of, 174, 237 
Astigmatic clock-face, 170 

fan, 171 
Asvmmetrv of cornea, 157 
Atropine, 60, 83, 126, 170, 196 

in astigmatism, 170 

in myopia, 151 

in retinoscopy, 83 

iu hvpermetropia, 126 
Axial line, '122, 138 

hypermetropia, 122 

myopia, 138 
Axis, optic, 201 

principal, 9, 12 

secondary, 12, 15 

visual, 201 

B 

Bar reading, 221 
Biconcave lenses, 11, 16, 20, 236 
Biconvex lenses, 11, 18, 236 
Binocular accommodation, 40 
vision, 211, 219 



Capsule of lens, 34 
Cardinal points, 23 
Caiaract, 148,164 

in myopia, 148 
Cases, retinoscopy, 110, 242 



266 



INDEX 



Cases, others, 242 

Centre of motion of the eye, 24 

optical, 12 
Choroid, thinning of, in myopia, 147 
Ciliary muscle, function of, 34 

in hypermetropia, 123 
in myopia, 145 

body, 34 
Civil Service, regulations for, 259 
Cob ii, 142 
Cocaine, 84 
Compound hypermetropic astigmatism, 161 

myopic astigmatism, 161 

svstem, points of, 23 
Concave lenses, 11, 20, 236 

mirror, in retinoscopy, 82 
Concomitant squint, 200 
Conjugate focus, 4, 14, 136 
Conjunctiva, 126, 224 
Convergence, 41 

amplitude of, 44 

insufficiency of, 228 

latent, 228 

metrical angle of, 44 

punctum proximum of, 44 

punctum remotum of, 44 

range of, 44 

relative, 50 
Convergent strabismus, 124, 208 
Cone, 56, 144 

of light, 158 
Convex lenses, 11, 18, 236 
Cornea, 22 

image formed on, 34 
Crescent, nivopir, 146 
Crystalline fens, 32, 122, 148, 157 
Cylindrical glasses, 32, 157, 1/3 



Decenteriug lenses, 194, 232 
Detachment of retina in myopia, 148 
Deviation, angle of, 9 

primary, 204 

secondary, 204 
Dioplre, 31, 257 
Dioptric svstem, 31 
Diplopia, 42, 211 
Direct ophthalmoscopic examination, 58, 

73 
Di3c, Placido's, 181 

shape of, in astigmatism, 172 
Distant type, 5fi 

Divergent strabismus, 146, 208, 215 
Divergence, appearance of, 123, 201 

latent, 228 
Donders, 121, 188,225 



Educational treatment of squint, 217 
Eltisticity ol capsule, 34 
of lens, 31, 40 

diminution with aye, 40, 187 
Elongation of eyeball in myopia, 138 



Emergent ray, 7 

Emmetropia, 26 

punctum proximum in, 28, 35 
punctum remotum in, 28, 35 

Erect image, 53, 73 

Erismann, 142 

Eserine, 197 

Esophoria, 229 

Exercises, orthoptic, 220 

Exnphoria, 229 

Eye, 21 

refracting media of, 22 
refracting surfaces of, 22 



Face, asymmetry of, in astigmatism, 53, 1 64 

in hypermetropia, 53, 124 

in myopia, 53 
Far point, see punctum remotum, 28, 34 
Focal length, 31 

interval, 160 

points, 24 
Focus, anterior, 10 

conjugate, 4, 14, 136 

principal, 3, 5, 9, 14, 24 
Formation ot images, 17 

by the eye, 25 
Fundus, 146 

G 
Glaucoma, 130, 195 
Glasses, 236 

biconcave.il, 16,20,236 

biconvex, 11, 18, 236 

cylindrical, 32, 157, 173 

orthoscopic, 194 

pantoscopic, 240 

prismatic, 240 

spherical, 31 

stenopaic, 157- 240 

tinted, 240 
Goggles, 241 

H 

Hereditary tendency in myopia, 141 
Herring's drop test, 222 
lleterophoiia, 228 
lloinatmpi.ic, 81, 102 
Homonymous images, 214, 230 
Hypermetropia, 26, 59, 117 

absolute, 121 

acquired, 128 

amount of, 126 

angle a in, 41, 123,150, 201 

axial, 122 

causes Of, 121 

diagnosis of, 126 

estimation of, 59, 126 

facultative, 121 

latent, 59, 121 

length of eyeball in, 122 

manifest, 59, 121 

original, 12h 

relative, 121 



INDEX 



267 



Hypermetropia, spectacles for, 129, 236 

symptoms of, 124 

tests for, 126 

treatment for, 129, 236 
Hypermetropic astigmatism, simple, 161 

compound, 161 
Hyperphoria, 229 

I 
Images, crossed, 64, 230 

formation of, 17 

homonymous, 64, 214, 230 

in astigmatism, 77 

in emmetropia, 75 

in hypermetropia, 76 

in myopia, 77 

on cornea, 34 

on lens, 34 

projected, 67 

real, 18 

virtual, 3,19 
Indian Services, regulations for, 259 
Indirect ophthalmoscopic examination, 

53, 66 
Insufficiency of convergence, 228 

test for, 230 
Internal recti, 146, 219 
Interval of Sturm, 160 

focal, 160 
Inverted image, 25 

Inverted ophthalmoscopic images, 53, 66 
Inversion of images by lenses, 18 

by the eye, 25 
Iris in accommodation, 34 

in hypermetropia, 123 

in myopia, 146 
Irregular astigmatism, 157 



Jackson, Dr, 104 

Jaeger, test type, 61 

Javal and Schiotz ophthalmometer, 176 



Lachrymal apparatus, 126 

Landolt, 154, 221 

Latent convergence, 228 

deviation in near vision, 231 
divergence, 229 
hypermetropia, 59, 121 

Length of eyeball, 22 
focal, 31 

in hypermetropia, 122 
in myopia, 138 

Lens, crystalline, 34, 122, 148, 157 

Lenses, 11, 31,236, 257 

biconcave, 11,16, 20, 236 
biconvex, 11, 18, 236 
conjugate focus, 4, 14, 136 
converging, 12 
cvlindrical, 32, 167,171 
d'ecentred, 194, 232 



Lenses, diverging, 12 

foci of, 14, 16 

images formed by, 17, 19 

influence of, on the size of the 
image, 237 

orthoscopic, 191 

principal focus, 5, 14, 24 

refraction by, 11 

spherical, '61 

table for presbyopia, 191 
Light, artificial, 148 
Lang sight, see presbyopia, 28, 187 

M 

Macula, 29, 55, 147 
Maddock's rod test, 46, 228 
Manifest hypermetropia, 59, 121 
Medium, refraction by, 7 
Meniscus, 11 
Metrical aujile, 44 

system of lenses, 31, 257 
Microphthalmos, 123 
Mirror, concave, for retinoscopy, 82 

plane, for retinoscopy, 1U2 

reflection by a plane, 2 
from a concave, 3 
from a convex, 6 
Mixed astigmatism, 161 
Monolateral strabismus, 206 
Movements of mirror in retinoscopy, 83 
Muscse volitantes, 145 
Muscle, ciliary, 34, 123, 145 

iris, 34, 123, 146 
Muscular asthenopia, 146, 227 
Myopia, 27, 134 

axial, 138 

causes of, 138 

determining causes, 141 

diagnosis of, 148 

estimation of degree, 148 

formation of image in, 136 

length of eyeball in, 138 

ophtlialniobcooic appearances in, 
146 

posterior staphyloma in, 138 

progressive, 135 

stationary, 153 

statistics in, 142 

symptoms of, 144 

treatment for, 150 
Myopic astigmatism, 161 

crescent, 146 



N 

Nagel on convergence, 42 

Navy, regulations for, 259 

Near point (punctum proximum), 28, 35 

Nega'ive, angle a, 203 

Nerve, opiic, in hypermetropia, 124 

in myopia, 147 
Nodal points, 24 
Nordenson, statistics of, 178 



268 



INDEX 



Objective examination, 53 
Operative treatment of squint, 219 
Optics, Chap. I 
Optic axis, 201 

disc in myopia, 147 

nerve in hypernietropia, 124 
in myopia, 147 
Optical centre, 12 
Ophthalmo-dynamometer, 47 
Ophthalmological Congress, 31 
Ophthalmometer of Javal and Schiotz, 176 
Ophthalmoscope, 53, 66 

direct examination, 66, 73 

indirect examination, 66 
Ophthalmoscopic appearances, 146 
Optometer of Tweedy, 178 

wire, 35 
Original hypermetropia, 128 
Orthoptic exercises, 220 
Orthoscopic lenses, 194 



Paralysis of accommodation, 195 

causes of, 196 

treatment of, 196 
Pantoscopic spectacles, 240 
Perimeter, 207 
Periodic strabismus, 206 
Pin-hole test, 54 
Placido's disc, 181 
Plane mirror, 102 
Points, cardinal, 23 

nodal, 23 

principal, 23 
Position in myopia, 150 
Posterior staphyloma, 138 
Pray, test letters of, 168 
Presbyopia, 28, 187 

age at which it commences, 189 

definition of, 188 

glasses for, 134, 190 

svmptoms of, 190 

table for, 191 

treatment of, 190 
Principal angle, 9 

focus, 3, 5, 9, 14, 24 

points, 24 
Prismatic spectacles, 240 
Prisms, 8, 42, 240 

abducting, 45, 240 

adducting, 46, 240 

to test convergence, 42 
rristnospheres, 232 
Progressive myopia, 135 
Protectors, 236 

Public works, regulations for, 260 
Punctum proxitnum, 28,35 

in emmetropia, 35 
in hypermetropia, 38 
in myopia, 39, 137 

remotum, 28, 34, 115, 137 



Punctum remotum — 

in emmetropia, 34 
in hypermetropia, 8 
in myopia, 137 
Pupil in accommodation, 34 
in hypermetropia, 123 
in myopia, 146 



Railways, regulations for, 260 
Range of accommodation, 37 

convergence, 44 
Rays, 1 

incident, 7 

emergent, 7 
Recti, internal, 146, 219 
Reflection, 2 

by concave surface, 3 

by convex surface, 6 

by plane surface, 2 
Refraction, 6, 22 

diminution of, 128, 145 

estimation of, 52 

index of, 7 

by lenses, 11 

by plane surface, 6 

by prisms, 8 

by spherical surface, 9 

by the eye, 22 
Regulations for army, 259 

for civil service, 259 

for navy, 259 
Regular astigmatism, 156 
Relative accommodation, 40, 50 

convergence, 50 
Remotum punctum, 28, 34, 115, 137 
in emmetropia, 34 
in hypermetropia, 38 
in myopia, 137 
Retina, 21, 25, 147 
Retinal asthenopia, 233 
Retinal image, size of, in hvpermetropia, 
68 

in myopia, 69 
Retinoscopy, 52, 66, 82 

cases of, 110 

in astigmatism, 97, 173 

in hypermetropia, 128 

in myopia, 149 

mirror for, 82 

oblique movements in, 101 

plane mirror in, 102 

rate of movement in, 90 
Rods and cones, 56, 144 
Rod test, 46, 228 



Scale for testing deviation, 231 

Schemer, 194 

Scheiner, 36, 64 

Scotomata, 144 

Secondary changes in myopia, 147 



INDEX 



269 



Shadows in retinoscopy, 82 

Shadow test, 82 

Snellen, 56 

Short sight (myopia), 27, 134 

Spasm of accommodation, 197 

causes of, 198 
symptoms of, 198 
treatment for, 199 
Spectacles (see also glasses), 236 

for aphakia, 133 

for astigmatism, 174, 237 

for hypermetropia, 129, 236 

for myopia, 152, 236 

for presbyopia, 190 

for strabismus, 217 
Simple hypermetropic astigmatism, 161 

myopic astigmatism, 161 
Squint, see strabismus, 200 
Staphyloma, posterior, 138 
Stationary myopia, 153 
Statistics in myopia, 147 
Stenopaic slit, 181 

glasses, 157, 240 
Stereoscope, 220 
Strabismometer, 206 
Strabismus, 200 

alternating, 206 

angle of, 207 

apparent, 200 

concomitant, 205 

constant, 206 

convergent, 124, 208 

divergent, 146, 208, 215 

monolateral, 206 

paralytic, 200 

periodic, 206 

real, 200 

treatment of, 217 
Sturm, interval of, 160 
Surfaces, refracting, of the eye, 22 
Symptoms of asthenopia, 225 

astigmatism, 164 

hypermetropia, 124 

myopia, 144 

presbyopia, 190 



Table for presbyopia, 191 

of amplitude of accommodation, 41 

of angles of convergence, 48 

of inches and dioptres, 258 

of length of axial line in hyperme- 
tropia, 122 
in myopia, 138 
Tenotomy, 219 
Test for aphakia, 132 

clock-face, 170 

fan, 171 

letters, Pray's, 168 

pin-hole, 54 

types, for near vision, 261 
Jaeger, 61 
Snellen, 56 
Treatment of asthenopia, 226, 232, 235 

astigmatism, 174, 237 

hypermetropia, 129, 2i56 

myopia, 150, 236 

paralysis of accommodation, 193 

presbyopia, 190 

spasm of accommodation, 199 

strabismus, 217 
Tweedy's optometer, 178 



Virtual focus, 5 

images, 3, 17 

Vision, acuteness of, 29, 54 
binocular, 211, 219 
in astigmatism, 165 
in hypermetropia, 126 
in myopia, 148 

Visual angle, 29, 56 
axis, 201 

Vitreous, 148 



Yellow spot, 29, 55, 147 
Young, 157 



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Aural Disease. 128 Illustrations. 2d Edition. Just Ready. $5.50 

PRITCHARD. Diseases of the Ear. 3d Edition, Enlarged. 
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WOAKES. Deafness, Giddiness, and Noises in the Head. 
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tions. 2d Edition. #1.00 

HEDLEY. Therapeutic Electricity and Practical Muscle 
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GOULD AND PYLE. Compend of Diseases of the Eye and 
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Cloth, .80; Interleaved, #1.00 

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JESSOP. Manual of Ophthalmic Surgery and Medicine. Col- 
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10 SUBJECT CATALOGUE. 

MORTON. Refraction of the Eye. Its Diagnosis and the Cor- 
rection of its Errors. 6th Edition. $1.00 

OHLEMANN. Ocular Therapeutics. Authorized Translation, 
and Edited by Dr. Charles A. Oliver. $i .75 

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SWANZY. Diseases of the Eye and Their Treatment. 7th 

Edition, Revised and Enlarged. 164 Illustrations, 1 Plain Plate, 
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THORINGTON. Refraction and How to Refract. 200 Illustra- 
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FEVERS. 

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HEART. 

SANSOM. Diseases of the Heart. The Diagnosis and Pathology 
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THORNE. The Schott Methods of the Treatment of Chronic 
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HISTOLOGY. 

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STIRLING. Outlines of Practical Histology. 368 Illustrations. 

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STOHR. Histology and Microscopical Anatomy. Edited by 
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MEDICAL BOOKS. 



HYGIENE AND WATER ANALYSIS. 

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COPLIN. Practical Hygiene. A Complete American Text-Book. 
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ERNST AND ABRAMS. Prophylaxis and Personal Hygiene. 

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LEFFMANN. Analysis of Milk and Milk Products. Illus- 
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LINCOLN. School and Industrial Hygiene. .40 

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NOTTER. The Theory and Practice of Hygiene. 15 Plates 
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STEVENSON AND MURPHY. A Treatise on Hygiene. By 
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WEYL. Sanitary Relations of the Coal-Tar Colors. Author- 
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HARRIS AND BEALE. Treatment of Pulmonary Consump- 
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KNOPF. Pulmonary Tuberculosis. Its Modern Prophylaxis 
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POWELL. Diseases of the Lungs and Pleurae, including 
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MASSAGE. 

KLEEN. Hand-Book of Massage. Authorized translation by 
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MITCHELL AND GULICK. Mechanotherapy. Illus. In Press. 
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MATERIA MEDICA AND THERA- 
PEUTICS. 

BIDDLE. Materia Medica and Therapeutics. Including Dose 
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BRACKEN. Outlines of Materia Medica and Pharmacology. $2.75 

COBLENTZ. The Newer Remedies. Including their Synonyms, 
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3d Edition, Enlarged and Revised. $1.00 

COHEN. Physiologic Therapeutics. Mechanotherapy, Mental 
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DAVIS. Materia Medica and Prescription Writing. ^1.50 

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HELLER. Essentials of Materia Medica, Pharmacy, and 
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MAYS. Theine in the Treatment of Neuralgia. % bound, .50 

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MEDICAL BOOKS. 13 



SAYRE. Organic Materia Medica and Pharmacognosy. An 

Introduction to the Study of the Vegetable Kingdom and the Vege- 
table and Animal Drugs. Comprising the Botanical and Physical 
Characteristics. Source, Constituents, and Pharmacopeial Prepara- 
tions, Insects Injurious to Drugs, and Pharmacal Botany. With 
sections on Histology and Microtechnique, by W. C. Stevens. 
374 Illustrations, many of which are original. 2d Edition. 

Cloth, $4.50 

WHITE AND WILCOX. Materia Medica, Pharmacy, Phar- 
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Medicine and Therapeutics at the New York Post-Graduate Medical 
School. Cloth, $3. 00; Leather, $3. 50 

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the best book on Materia Medica and Therapeutics to place in the 
hands of students, and the practitioner will find it a most satisfactory 
work for daily use." — The Cleveland Medical Gazette. 



MEDICAL JURISPRUDENCE AND 
TOXICOLOGY. 

REESE. Medical Jurisprudence and Toxicology. A Text-Book 

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American Journal of the Medical Sciences. 

MANN. Forensic Medicine and Toxicology. Illus. $6.50 

TANNER. Memoranda of Poisons. Their Antidotes and Tests. 
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MICROSCOPY. 

CARPENTER. The Microscope and Its Revelations. 8th 

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REEVES. Medical Microscopy, including Chapters on Bacteri- 
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WETHERED. Medical Microscopy. A Guide to the Use of the 

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SUBJECT CATALOGUE. 



MISCELLANEOUS. 

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GOWERS. Syphilis and the Nervous System. $1.00 



MEDICAL BOOKS. 16 



GOWERS. Clinical Lectures. A New Volume of Essays on the 
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GOWERS. Epilepsy and Other Chronic Convulsive Diseases. 
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NURSING (see also Massage). 

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Edition. 25 Illustrations. $1.00 

TEMPERATURE AND CLINICAL CHARTS. See page 6. 

VOSWINKEL. Surgical Nursing. Second Edition, Enlarged. 
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SUBJECT CATALOGUE 



OBSTETRICS. 

CAZEAUX AND TARNIER. Midwifery. With Appendix by 
Mundb. The Theory and Practice of Obstetrics, including the Dis- 
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DAVIS. A Manual of Obstetrics. 3d Edition. Preparing. 

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FULLERTON. Obstetric Nursing. 5th Ed. Illustrated. $1.00 

LANDIS. Compend of Obstetrics. 6th Edition, Revised by Wm. 
H. Wells, Assistant Demonstrator of Clinical Obstetrics, Jefferson 
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WINCKEL. Text-Book of Obstetrics, Including the Pathol- 
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Translation by J. Clifton Edgar, a.m., m.d. With nearly 200 Illus- 
trations. Cloth, $5.00 ; Leather, $6.00 



PATHOLOGY. 

BARLOW. General Pathology. 795 pages. 8vo. $5.00 

BLACK. Micro-Organisms. The Formation of Poisons. .75 

BLACKBURN. Autopsies. A Manual of Autopsies Designed foi 
the Use of Hospitals for the Insane and other Public Institutions. 
Ten full-page Plates and other Illustrations. %\ .25 

COPLIN. Manual of Pathology. Including Bacteriology, Technic 
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Just Ready. $3-5° 

DA COSTA. Clinical Pathology of the Blood. Il.us. In Press. 

HEWLETT. Manual of Bacteriology. 7, Illustrations. $3.00 

THAYER. Compend of General Pathology. Illustrated. 

Preparing. 

VIRCHOW, Post-Mortem Examinations. A Description and 
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of the Berlin Charity Hospital, with Special Reference to Medico- 
Legal Practice. 3d Edition, with Additions. .75 

WHITACRE. Laboratory Text-Book of Pathology. With 
121 Illustrations. $ I -5° 

WILLIAMS. Bacteriology. A Manual for Students. 78 Illus- 
trations. $1-50 

PHARMACY. 

Special Catalogue of Books on Pharmacy sent free upon application. 

COBLENTZ. Manual of Pharmacy. A Complete Text-Book 
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BEASLEY. Book of 3100 Prescriptions. Collected from the 
Practice of the Most Eminent Physicians and Surgeons — English, 
French, and American. A Compendious History of the Materia 
Medica, Lists of the Doses of all the Officinal and Established Pre- 
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MEDICAL BOOKS. 17 

BEASLEY. Druggists' General Receipt Book. Comprising 
a Copious Veterinary Formulary, Recipes in Patent and Proprietary 
Medicines, Druggists' Nostrums, etc. ; Perfumery and Cosmetics, 
Beverages, Dietetic Articles and Condiments, Trade Chemicals, 
Scientific Processes, and many Useful Tables. 10th Ed. $2.00 

BEASLEY. Pharmaceutical Formulary. A Synopsis of the 
British, French, German, and United States Pharmacopoeias. Com- 
prising Standard and Approved Formulae for the Preparations and 
Compounds Employed in Medicine. 12th Edition. $2.00 

PROCTOR. Practical Pharmacy. Lectures on Practical Phar- 
macy. With Wood Engravings and 32 Lithographic Fac-simile 
Prescriptions. 3d Edition, Revised, and with Elaborate Tables of 
Chemical Solubilities, etc. #3.00 

ROBINSON. Latin Grammar of Pharmacy and Medicine. 
3d Edition. With elaborate Vocabularies. $i-75 

SAYRE. Organic Materia Medica and Pharmacognosy. An 
Introduction to the Study of the Vegetable Kingdom and the Vege- 
table and Animal Drugs. Comprising the Botanical and Physical 
Characteristics, Source, Constituents, and Pharmacopeial Prepar- 
ations, Insects Injurious to Drugs, and Parmacal Botany. With 
sections on Histolqgv and Microtechnique, by W. C. Stevens. 
374 Illustrations. Second Edition. Cloth, $4.50 

SCOVILLE. The Art of Compounding. Second Edition, Re- 
vised and Enlarged. Cloth, $2.50 

STEWART. Compend of Pharmacy. Based upon " Reming- 
ton's Text-Book of Pharmacy." 5th Edition, Revised in Accord- 
ance with the U. S. Pharmacopoeia, 1890. Complete "Tables of 
Metric and English Weights and Measures. .80; Interleaved, $1.00 

UNITED STATES PHARMACOPOEIA. 7th Decennial Revision. 
Cloth, $2. 50 (postpaid, $2.77) ; Sheep, $3.00 (postpaid, $3.27) ; Inter- 
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Select Tables from the U. S. P. Being Nine of the Most Impor- 
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POTTER. Hand-Book of Materia Medica, Pharmacy, and 
Therapeutics. 600 Prescriptions. 7th Ed. Clo.,^5.00; Sh.,$6.oo 

PHYSICAL DIAGNOSIS. 

BROWN. Medical Diagnosis. A Manual of Clinical Methods. 
4th Edition. 112 Illustrations. Cloth, #2.25 

DA COSTA. Clinical Examination of the Blood. Illustrated. 

In Press. 

FENWICK. Medical Diagnosis. 8th Edition. Rewritten and 
very much Enlarged. 135 Illustrations. Cloth, $2.50 

MEMMINGER. Diagnosis by the Urine. 2dEd. 24 Illus $1.00 

TYSON. Hand-Book of Physical Diagnosis. For Students and 
Physicians. By the Professor of Clinical Medicine in the University 
of Pennsylvania. Illus. 3d Ed., Improved and Enlarged. With 
Colored and other Illustrations. $150 

PHYSIOLOGY. 

BIRCH. Practical Physiology. An Elementary Class Book. 

62 Illustr itions. $1.75 

BRUBAKER. Compend of Physiology. 10th Edition, Revised 

and Enlarged. Illustrated. Jttst Ready. .80; Interleaved, $1.00 

2 



18 SUBJECT CATALOGUE. 

KIRKES. Physiology. (16th Authorized Edition. Dark-Red Cloth.) 
A Hand-Book of Physiology. 16th Edition, Revised, Rearranged, 
and Enlarged. By Prof. W. D. Halliburton, of Kings College, 
London. 671 Illustrations, some of which are printed in colors. 
Just Ready. Cloth, $3.00; Leather, $3.75 

LANDOIS. A Text-Book of Human Physiology, Including 
Histology and Microscopical Anatomy, with Special Reference to 
the Requirements of Practical Medicine. 5th American, translated 
from the 9th German Edition, with Additions by Wm. Stirling, 
M.D.,D.sc. 845 Illus., many of which are printed in colors. In Press. 

STARLING. Elements of Human Physiology. 100 Ills. $1.00 

STIRLING. Outlines of Practical Physiology. Including 
Chemical and Experimental Physiology, with Special Reference to 
Practical Medicine. 3d Edition. 289 Illustrations. $2.00 

TYSON. Cell Doctrine. Its History and Present State. $1.50 

PRACTICE. 

BEALE. On Slight Ailments; their Nature and Treatment. 

2d Edition, Enlarged and Illustrated. $1.25 

FOWLER. Dictionary of Practical Medicine. By various 
writers. An Encyclopaedia of Medicine. Clo.,$3.oo; Half Mor. #4.00 
GOULD AND PYLE. Cyclopedia of Practical Medicine and 
Surgery. A Concise Reference Handbook, Alphabetically 
Arranged, with particular Reference to Diagnosis and Treatment. 
Edited .by Drs. Gould and Pyle, Assisted by 72 Special Con- 
tributors. Illustrated, one volume. Large Square Octavo, Uniform 
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HUGHES. Compend of the Practice of Medicine. 6th Edition, 
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Part I. Continued, Eruptive, and Periodical Fevers, Diseases of the 
Stomach, Intestines, Peritoneum, Biliary Passages, Liver, Kid- 
neys, etc., and General Diseases, etc. 
Part II. Diseases of the Respiratory System, Circulatory System, 
and Nervous System; Diseases of the Blood, etc. 

Price of each part, .80; Interleaved, $1.00 
Physician's Edition. In one volume, including the above two 
parts, a Section on Skin Diseases, and an Index. 6th Revised 
Edition. 625 pp. Just Ready. Full Morocco, Gilt Edge, $2.25 
TAYLOR. Practice of Medicine. 5th Edition. Cloth, $4.00 

TYSON. The Practice of Medicine. By James Tyson, m.d., 
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Treatment. 2d Edition, Enlarged and Revised. Colored Plates and 
125 other Illustrations. 1222 Pages. 8vo. Just Ready. 

Cloth, $5.50; Leather, $6.50 

PRESCRIPTION BOOKS. 

BEASLEY. Book of 3100 Prescriptions. Collected from the 
Practice of the Most Eminent Physicians and Surgeons— English, 
French, and American. A Compendious History of the Materia, 
Medica, Lists of the Doses of all Officinal and Established Prepara- 
tions, and an Index of Diseases and their Remedies. 7th Ed. $2.00 



MEDICAL BOOKS. 



BEASLEY. Druggists' General Receipt Book. Comprising 
a Copious Veterinary Formulary, Recipes in Patent and Proprie- 
tary Medicines, Druggists' Nostrums, etc. ; Perfumery and Cos- 
metics, Beverages, Dietetic Articles and Condiments, Trade Chem- 
icals, Scientific Processes, and an Appendix of Useful Tables. 
10th Edition, Revised. #2.00 

BEASLEY. Pocket Formulary. A Synopsis of the British, French, 
German, and United States Pharmacopoeias and the chief unofficial 
Formularies. 12th Edition. $2.00 

SKIN. 

BULKLEY. The Skin in Health and Disease. Illustrated. .40 
CROCKER. Diseases of the Skin. Their Description, Pathol- 
ogy, Diagnosis, and Treatment, with Special Reference to the Skin 
Eruptions of Children. 92 Illus. 3d Edition. Preparing. 

IMPEY. Leprosy. 37 Plates. 8vo. $3.50 

SCHAMBERG. Diseases of the Skin. 2d Edition, Revised and 
Enlarged. 105 Illustrations. Being No. 16 ? Quiz-Compend? Series. 
Just Ready, Cloth, .80 ; Interleaved, #1.00 

VAN HARLINGEN. On Skin Diseases. A Practical Manual 
of Diagnosis and Treatment, with special reference to Differential 
Diagnosis. 3d Edition, Revised and Enlarged. With Formulae 
and 60 Illustrations, some of which are printed in colors. $2.75 

SURGERY AND SURGICAL DIS- 
EASES (see also Urinary Organs). 

BUTLIN.. Operative Surgery of Malignant Disease. 2d Edi- 
tion. Illustrated. Octavo. Just Ready. $4-5o 
CRIPPS. Ovariotomy and Abdominal Surgery. Illus. #8.00 
DEAVER. Surgical Anatomy. A Treatise on Human Anatomy 
in its Application to Medicine and Surgery. With about 400 very 
Handsome full-page Illustrations Engraved from Original Drawings 
made by special Artists from Dissections prepared for the purpose. 
Three Volumes. Royal Square Octavo. 

Cloth, $21.00 ; Half Morocco or Sheep, $24.00 ; Half Russia, $27.00 
Complete descriptive circular and special terms upon application. 

DEAVER. Appendicitis, Its Symptoms, Diagnosis, Pathol- 
ogy, Treatment, and Complications. Elaborately Illustrated 
with Colored Plates and other Illustrations. 2d Edition. $3. 50 

DULLES. What to Do First in Accidents and Poisoning. 
5th Edition. New Illustrations. $1.00 

FULLERTON. Surgical Nursing. 3d Edition. 69 Illus. $100 

HAMILTON. Lectures on Tumors. 3d Edition. $1.25 

HEATH. Minor Surgery and Bandaging, nth Ed., Revised 
and Enlarged. 176 Illustrations, Formulas, Diet List, etc. $1.25 

HEATH. Injuries and Diseases of the Jaws. 4th Ed. $4.50 
HORW1TZ. Compend of Surgery and Bandaging, including 
Minor Surgery, Amputations, Fractures, Dislocations, Surgical Dis- 
eases, and the Latest Antiseptic Rules, etc., with Differential Diagno- 
sis and Treatment. 5th Edition, very much Enlarged and Rear- 
ranged. 167 Illustrations, 98 Formulae. Clo., .80 ; Interleaved, $1.00 



20 SUBJECT CATALOGUE. 

JACOBSON. Operations of Surgery. Over 200 Illustrations. 

Cloth, $3.00 ; Leather, $4.00 
JACOBSON. Diseases of the Male Organs of Generation. 

88 Illustrations. $6.00 

LANE. Surgery of the Head and Neck, no Illustrations. 
2d Edition. $5-oo 

MACREADY. A Treatise on Ruptures. 24 Full-page Litho- 
graphed Plates and Numerous Wood Engravings. Cloth, $6.00 
MAYLARD. Surgery of the Alimentary Canal. 97 Illustrations. 
2d Edition, Revised. Just Ready. $3.00 

MOULLIN. Text-Book of Surgery. With Special Reference to 
Treatment. 3d American Edition. Revised and edited by John B. 
Hamilton, m.d., ll.d., Professor of the Principles of Surgery and 
Clinical Surgery, Rush Medical College, Chicago. 623 Illustrations, 
over 200 of which are original, and many of which are printed in 
colors. Handsome Cloth, $6.00; Leather, $7. 00 

ROBERTS. Fractures of the Radius. A Clinical and Patho- 
logical Study. 33 Illustrations. $1.00 

SMITH. Abdominal Surgery. Being a Systematic Description cf 
all the Principal Operations. 224 Illus. 6th Ed. 2 Vols. Clo., $10.00 

SWAIN. Surgical Emergencies. Fifth Edition. Cloth, $1.75 

VOSWINKEL. Surgical Nursing. Second Edition, Revised and 
Enlarged, in Illustrations. $1.00 

WALSHAM. Manual of Practical Surgery. 7th Ed., Re- 
vised and Enlarged. 483 Engravings. 950 pages. Just Ready. $3.50 



THROAT AND NOSE (see also Ear). 

COHEN. The Throat and Voice. Illustrated. .40 

HALL. Diseases of the Nose and Throat. Two Colored 
Plates and 59 Illustrations. New Edition Preparing. 

HOLLOPETER. Hay Fever. Its Successful Treatment. $1.00 

KNIGHT. Diseases of the Throat. A Manual for Students. 
Illustrated. Nearly Ready. 

LAKE. Laryngeal Phthisis. Colored Illustrations. In Press. 

MACKENZIE. Pharmacopoeia of the London Hospital for 
Dis. of the Throat. 5th Ed., Revised by Dr. F. G. Harvey. $1.00 

McBRIDE. Diseases of the Throat, Nose, and Ear. With col- 
ored Illus. from original drawings. 3d Ed. hist Ready $7°° 

POTTER. Speech and its Defects. Considered Physiologically, 
Pathologically, and Remedially. $1.00 

SHEILD. Nasal Obstructions. Illustrated. In Press. 



URINE AND URINARY ORGANS. 

ACTON. The Functions and Disorders of the Reproductive 
Organs in Childhood, Youth, Adult Age, and Advanced Life. 
Considered in their Physiological, Social, and Moral Relations. 
8th Edition. *i-75 



MEDICAL BOOKS. 



BEALE. One Hundred Urinary Deposits. On eight sheets, 
for the Hospital, Laboratory, or Surgery. Paper, #2.00 

HOLLAND. The Urine, the Gastric Contents, the Common 
Poisons, and the Milk. Memoranda, Chemical and Microscopi- 
cal, for Laboratory Use. Illustrated and Interleaved. 6th Ed. $1.00 

JACOBSON. Male Organs of Generation. 88 Illus. $6.00 

KEHR. Gall-Stone Disease. Translated by William Watkyns 
Seymour, m d. In Press. 

KLEEN. Diabetes and Glycosuria. $2.50 

MEMMINGER. Diagnosis by the Urine. 2d Ed. 24 Illus. gi.oo 

MORRIS. Renal Surgery, with Special Reference to Stone in the 
Kidney and Ureter and to the Surgical Treatment of Calculous 
Anuria. Illustrated. $2.00. 

MOULLIN. Enlargement of the Prostate. Its Treatment and 
Radical Cure. 2d Edition. Illustrated. Just Ready. $ x -75 

MOULLIN. Inflammation of the Bladder and Urinary Fever. 
Octavo. $1.50 

SCOTT. The Urine. Its Clinical and Microscopical Examination. 
41 Lithographic Plates and other Illustrations. Quarto. Cloth, $5.00 

TYSON. Guide to Examination of the Urine. For the Use of 
Physicians and Students. With Colored Plate and Numerous Illus- 
trations engraved on wood. 9th Edition, Revised. $1-25 

VAN NUYS. Chemical Analysis of Urine. 39 Illus. $1.00 

VENEREAL DISEASES. 

COOPER. Syphilis. 2d Edition, Enlarged and Illustrated with 

20 full-page Plates. $5.00 

GOWERS. Syphilis and the Nervous System. 1.00 

STURGIS AND CABOT. Student's Manual of Venereal 

Diseases. 7th Revised and Enlarged Ed i2mo. Just Ready. #1.25 

VETERINARY. 

BALLOU. Veterinary Anatomy and Physiology. 29 Graphic 
Illustrations. .80; Interleaved, $1.00 

TUSON. Veterinary Pharmacopoeia. Including the Outlines of 
Materia Medica and Therapeutics. 5th Edition. #2.25 

WOMEN, DISEASES OF. 

BISHOP. Uterine Fibromyomata. Illustrated. In Press. 

BYFORD (H. T.). Manual of Gynecology. Second Edition, 
Revised and Enlarged by 100 pages. 341 Illustrations. #3.00 

DUHRSSEN. A Manual of Gynecological Practice. 105 
Illustrations. $1.50 

FULLERTON. Surgical Nursing. 3d Edition, Revised and 
Enlarged. 69 Illustrations. $1.00 

LEWERS. Diseases of Women. 146 Illus. 5th Ed. #2.50 

MONTGOMERY. Practical Gynecology. A Complete Sys- 
tematic Text-Book. 527 Handsome Illustrations. 8vo. Just Ready. 

Cloth, $5.00; Leather, $6.00 

WELLS. Compend of Gynecology. Illustrated. 2d Edition. 

.80; Interleaved, 1 1. 00 



22 SUBJECT CATALOGUE. 

COMPENDS. 



From The Southern Clinic. 

" We know of no series of books issued by any house that so fully 
meets our approval as these ?Quiz-Compends?. They are well ar- 
ranged, full, and concise, and are really the best line of text-books that 
could be found for either student or practitioner." 



BLAKISTON'S ?QUIZ-COMPENDS? 

The Best Series of Manuals for the Use of Students. 
Price of each, Cloth, .80. Interleaved, for taking Notes, $1.00. 

jJSJ- These Compends are based on the most popular text-books 
and the lectures of prominent professors, and are kept constantly re- 
vised, so that they may thoroughly represent the present state of the 
subjects upon which they treat. 

4®=* The authors have had large experience as Quiz-Masters and 
attaches of colleges, and are well acquainted with the wants of students. 

Jgg~ They are arranged in the most approved form, thorough and 
concise, containing over 6oo fine illustrations, inserted wherever they 
could be used to advantage. 

jfl®~ Can be used by students of any college. 

*S» They contain information nowhere else collected in such a 
condensed, practical shape. Illustrated Circular free. 

No. i. POTTER. HUMAN ANATOMY. Sixth Revised and 
Enlarged Edition. Including Visceral Anatomy. Can be used 
with either Morris's or Gray's Anatomy. 117 Illustrations and 16 
Lithographic Plates of Nerves and Arteries, with Explanatory 
Tables, etc. By Samuel O. L. Potter, m.d., Professor of the 
Practice of Medicine, College of Physicians and Surgeons, San 
Francisco ; Br'gade Surgeon, U. S. Vol. 

No. 2. HUGHES. PRACTICE OF MEDICINE. Part I. Sixth 
Edition, Enlarged and Improved. By Daniel E. Hughes, m.d., 
Physician-in-Chief, Philadelphia Hospital, late Demonstrator of 
Clinical Medicine, Jefferson Medical College, Phila. 

No. 3. HUGHES. PRACTICE OF MEDICINE. Part II. 
Sixth Edition, Revised and Improved. Same author as No. 2. 

No. 4. BRUBAKER. PHYSIOLOGY. Tenth Edition, with 
Illustrations and a table of Physiological Constants. Enlarged 
and Revised. By A. P. Brubaker, m.d., Professor of Physiology 
and General Pathology in the Pennsylvania College of Dental 
Surgery; Adjunct Professor of Physiology, Jefferson Medical 
College, Philadelphia, etc. 

No. 5. LANDIS. OBSTETRICS. Sixth Edition. By Henry G. 
Landis, m.d. Revised and Edited by Wm. H. Wells, m.d., 
Instructor of Obstetrics, Jefferson Medical College, Philadelphia. 
Enlarged. 47 Illustrations. 

No. 6. POTTER. MATERIA MEDICA, THERAPEUTICS, 
AND PRESCRIPTION WRITING. Sixth Revised Edition 
(U. S. P. 1890). By Samuel O. L. Potter, m.d., Professor of 
Practice, College of Physicians and Surgeons, San Francisco; 
Brigade Surgeon, U. S. Vol. 



MEDICAL BOOKS. 23 

PQUIZ-COMPENDS ?— Continued. 

No. 7. WELLS. GYNECOLOGY. Second Edition. ByW M .H. 
Wells, m.d., Instructor of Obstetrics, JeffersoD College, Philadel- 
phia. 140 Illustrations. 

No. 8. GOULD AND PYLE. DISEASES OF THE EYE 
AND REFRACTION. Second Edition. Including Treatment 
and Surgery, and a Section on Local Therapeutics. By George 
M. Gould, m.d., and W. L. Pyle, m.d. With Formulae, Glossary, 
Tables, and 109 Illustrations, several of which are Colored. 

No. 9. HORWITZ. SURGERY, Minor Surgery, and Bandag- 
ing. Fifth Edition, Enlarged and Improved. By Orvillb 
Horwitz, b. s., m.d., Clinical Professor of Genito- Urinary Surgery 
and Venereal Diseases in Jefferson Medical College ; Surgeon to 
Philadelphia Hospital, etc. With 98 Formulae and 71 Illustrations. 

No. 10. LEFFMANN. MEDICAL CHEMISTRY. Fourth 
Edition. Including Urinalysis, Animal Chemistry, Chemistry of 
Milk, Blood, Tissues, the Secretions, etc. By Henry Leffmann, 
m.d., Professor of Chemistry in Pennsylvania College of Dental 
Surgery and in the Woman's Medical College, Philadelphia. 

No. ix. STEWART. PHARMACY. Fifth Edition. Based upon 
Prof. Remington's Text-Book of Pharmacy. By F. E. Stewart, 
m.d., ph. g., late Quiz-Master in Pharmacy and Chemistry, Phila- 
delphia College of Pharmacy ; Lecturer at Jefferson Medical 
College. Carefully revised in accordance with the new U. S. P. 

No. 12. BALLOU. VETERINARY ANATOMY AND PHY- 
SIOLOGY. Illustrated. By Wm. R. Ballou, m.d., Professor 
of Equine Anatomy at New York College of Veterinary Surgeons ; 
Physician to Bellevue Dispensary, etc. 29 graphic Illustrations 

No. 13. WARREN. DENTAL PATHOLOGY AND DEN- 
TAL MEDICINE. Third Edition, Illustrated. Containing 
a Section on Emergencies. By Geo. W. Warren, d.d.s., Chief 
ot Clinical Staff, Pennsylvania College of Dental Surgery. 

No. 14. HATFIELD. DISEASES OF CHILDREN. Second 
Edition. Colored Plate. By Marcus P. Hatfield, Profes- 
sor of Diseases of Children, Chicago Medical College. 

No. 15. THAYER. GENERAL PATHOLOGY. By A. E. 

Thayer, m.d., Cornell University Medical College.. Illustrated. 

Preparing. 

No. 16. DISEASES OF THE SKIN. Second Edition. By 
Jay F. Schamberg, m.d., Professor of Diseases of the Skin, 
Philadelphia Polyclinic. Second Edition, Revised and Enlarged. 
105 handsome Illustrations. 

No. 17. CUSHING. HISTOLOGY. By H. H. Cushing, m.d., 
Demonstrator of Histology, Jefferson Medical College, Philadel- 
phia. Illustrated. Preparing. 

Price, each, Cloth, .80. Interleaved, for taking Notes, $1.00. 

Careful attention has been given to the construction of each sentence, 
and while the books will be found to contain an immense amount of 
knowledge in small space, they will likewise be found easy reading ; 
there is no stilted repetition of words ; the style is clear, lucid, and dis- 
tinct. The arrangement of subjects is systematic and thorough ; there 
is a reason for every word. They contain over 600 illustrations. 



Morris' 
Anatomy 

Second Edition, Revised and Enlarged. 

790 Illustrations, of which many 
are in Colors. 

Royal Octavo. Cloth, $6.00 ; Sheep, $7.00 ; 
Half Russia, $8.00. 



From The Medical Record, New York. 

11 The reproach that the English language can boast of no 
treatise on anatomy deserving to be ranked with the masterly 
works of Henle, Luschka, Hyrtl, and others, is fast losing 
its force. During the past few years several works of great 
merit have appeared, and among these Morris's "Anatomy " 
seems destined to take first place in disputing the palm in 
anatomical fields with the German classics. The nomencla- 
ture, arrangement, and entire general character resemble 
strongly those of the above-mentioned handbooks, while in 
the beauty and profuseness of its illustrations it surpasses 
them. . . . The ever-growing popularity of the book 
with teachers and students is an index of its value, and it 
may safely be recommended to all interested." 

From The Philadelphia Medical Journal. 

" Of all the text-books of moderate size on human anatomy 
in the English language, Morris is undoubtedly the most 
up-to-date and accurate." 



V Handsome Descriptive Circular, with Sample Pages and 
Colored Illustrations, will be sent free upon application. 



